The pillars of Islam are of two categories: verbal and physical.
As for the verbal pillar then it is the Testimony of Faith.
As for the physical then it is of three categories:
1-   Bodily: Fasting and Prayer
2-   Financial: Zakat
3-   Bodily and Financial: Pilgrimage

The speech of the heart and its action are conditions for all of those pillars.

The textual evidence that establishes these five pillars are numerous, and they are of two types: one that mention all of them collectively, and another that mentions a pillar specifically. From the evidence that encompassed them all is: the hadeeth of Jibreel (peace be upon him) which is related by ‘Umar ibn al-Khattaab (may Allah be pleased with him) and others, and in it he Jibreel says, “O’ Muhammad, inform me of Islam.” The Messenger of Allah ? said: “Islam is that you testify that there is no deity worthy of worship except Allah, and that Muhammad is the Messenger of Allah ? and you establish the prayer, give alms, fast Ramadan, and make pilgrimage to the House if you are capable of doing so.” [155] Reported by Muslim (8)

The Testimony of Faith
The Testimony of Faith is the first of these pillars, and it is the most important basis in the composition of Islam. It is the testification that there is no deity worthy of worship except Allah, and that Muhammad is the Messenger of Allah. A person cannot enter into Islam without it and does not leave it except by negating it; either out of denial of what it means, or out of arrogance towards that which it obligates. Due to this, the Messenger of Allah ? did not call to anything before it, and neither Allah nor the Messenger of Allah ? accept anything from anyone without it, for in the first part of the testimony a person knows Who is worshipped, and in the second one they know how to worship Him, and by what means they can reach Him. And how can anyone believe in the need to worship before knowing Who is being worshipped, and how can they carry out that worship if they do not know how Allah has commanded them to worship? In the first testimony, there is the oneness of the Worshipped Who did not create all of creation except that they worship Him alone without associating any partners to Him. Andin the second testimony, there is the oneness of the path, which one cannot reach Allah the Exalted except through it, and there is no religion that is accepted from the one who chooses a path other than it or prefers other than it. For the worship of Allah, the Exalted that He created all of creation to do and decreed that it should be directed solely to Him, is a command that includes everything that pleases Allah the Exalted, by belief, speech, or action. And knowledge of that which The Exalted loves and what pleases Him cannot be attained except by way of the legislation that He sent through His Messenger ? and revealed through His Book. Allah, Most High, says: Say, if you all love Allah then follow me, so that Allah will love you and forgive your sins, and Allah is the oft-Forgiving and most Merciful aal-‘Imraan :31

The Prayer
The second pillar is the establishing of the prayer with all of its conditions and rights. Allah, Most High, says: And establish the prayer, and give alms, and obey the Messenger so that you may be shown mercy. An-Noor :56. And in the hadeeth of Mu’aath ibn Jabal (may Allah be pleased with him) when he was sent by the Prophet ? to Yemen and was told, “Indeed you will arrive among a nation of People of the Book, so make the Two Testimonies of Islam  the first thing you call them to; that there is no deity worthy of worship except Allah, and that I am the Messenger of Allah. If they obey you in that, then explain to them that Allah has obligated upon them five prayers in every day and night. If they obey you in that, then explain to them that Allah has obligated upon them charity that is taken from their wealthy and given to their poor. And if they obey you in that then beware of taking the most valuable of their wealth, and fear the supplication of the oppressed, for there is no veil between it and Allah.” [156] Reported by al-Bukhaaree (1496), and Muslim (19)

The Companions (may Allah be pleased with them) and those that followed them were in agreement regarding the disbelief and execution of the one who forsakes the prayer out of denial of its obligation, or out of arrogance towards it, even if they say that there is no deity worthy of worship except Allah. Ibn Rushd said, “Whoever denies the obligation of prayer then they are a disbeliever and thus asked to repent, then they repent. If they do not then they are executed, and their wealth belongs to the Muslims, as is the case of the apostate who is killed upon their apostasy, according to the consensus of the people of knowledge, with there being no disagreement between them in this.” [157] “al-Muqaddimat al-Mumahhidat” (1/141) Ibn Taymiyyah said, “Here there are three categories, one is if they deny its obligation (i.e., the prayer) then they are a disbeliever by agreement of the scholars. The second is that they do not deny its obligation but abstain from doing it out of arrogance, enmity, or hatred towards Allah and His Messenger. So they would say, “I know that Allah has obligated it upon the Muslims, and the Messenger is trustworthy in his conveying of the Quran” but they abstain from doing the prayer; out of arrogance, or enmity towards the Messenger, or out of partisanship towards their own religion, or out of hatred for what the Messenger has come with; then this person likewise is a disbeliever by agreement, for Iblees did not forsake the prostration he was commanded with out of denial of its obligation, for Allah had directly commanded him; rather he refused and was arrogant, and so was from the disbelievers.” [158] “Majmoo’ al-Fataawa” (20/97)

As for whoever intentionally forsakes the prayer out of carelessness and laziness, are they a disbeliever or a Muslim? In this are two opinions from the people of knowledge, and with each group there are evidence and proofs. And an opinion that reconciles the evidences of the two opinions is that: The standard by which we ascertain that the forsaking of the prayer constitutes disbelief, is that it is the unrestricted forsaking of it that is realised through the complete abandoning of prayer, or with the continual abstaining from establishing it, or by forsaking it in the vast majority of times such that it appears to be the unrestricted form of forsaking it, in that the person is not considered as someone who establishes and carries out the prayer. And excommunication is not caused by the general act of leaving the prayer, such that it obligates us to excommunicate someone who leaves a single prayer. [159] “Majmoo’ al-Fataawa” (22/49) | “Majmoo’ Fataawa wa Rasa’il al- ‘‘Uthaymeen” (12/54) | “Dhawabit at-Takfir” – ‘Abdullah al-Qarni (p.159)

That is attested to by a few textual evidence, like:
1-   Allah, Most High, says: But then they were succeeded by a people who neglected the prayer and followed their desires. And so will they come to face the consequences of their evil. Maryam: 59
What is intended by the neglecting of the prayer here is abandoning it entirely, as was stated by Muhammad ibn Ka’b al-Quradee, Zayd ibn Aslam, and as-Suddee, which is also the view that was chosen by Ibn Jareer. [160] “Tafseer Ibn Jareer” (15/569) | “Tafseer Ibn Katheer” (5/243)
2-   The Prophet ? said: “Five prayers have been obligated by Allah upon His servants in a day and a night. So, whoever preserves them then for them is a promise from Allah that He will enter them into paradise. And whoever does not preserve them does not have that promise from Allah; if He wishes He will punish them, and if He wishes He will enter them into paradise.” [161] Reported by Abu Dawud (1420), an-Nasa’I (461), Ibn Maajah (1401), and Ahmad (22693), with slight variation from the hadeeth of ‘Ubadah ibn as-Samit (may Allah be pleased with him). Ibn Hibbaan authenticated it in his “Saheeh” (2417), as did Ibn ‘Abd al-Barr in “at-Tamheed” (23/288), Ibn al-‘Arabi in “’Aridhatul-Ahwadhi” (1/447), an-Nawawi in “al-Majmoo’” (4/20), Ibn al-Mulaqqin in “al-Badr al-Munir” (5/389), Al-Albaanee in “Saheeh Sunan Abi Dawud” (1420), and Shu’ayb al-Arna’oot in his edition of “Sunan Abi Dawud” (1420)

Ibn Taymiyyah said, “The Prophet ? included the one who does not preserve it as being subject to the will of Allah, not the one who abandons it. The essence of preservation requires that they do pray however they do not preserve it… for a lot of people, rather most of them, in many regions, are not people who preserve the five prayers, nor are they those who abandon them entirely. Rather they pray sometimes, and they neglect it at others. In these people there is Imaan and hypocrisy, and the apparent outward rulings of Islam apply to them.” [162] “Majmoo’ al-Fataawa” (7/615-617)

Obligatory Zakat
From the most important pillars of Islam is the giving of alms, and it is associated with the prayer in the book of Allah. Allah, Most High, says: And establish the prayer and give alms al-Baqarah :43, and He, Glorified be He, said: So if they repent and establish the prayer and give alms then they are your brothers in the religion. at-Tawbah :11 And whoever abstains out of rejection of its obligation then they are a disbeliever by consensus. And if they acknowledge its obligation and are a strong and powerful group, then the leader is to fight them. ‘Abdullah ibn ‘Umar (may Allah be pleased with them both said, “The Messenger of Allah ? said, “I have been commanded to fight the people until they bear witness that there is no deity worthy of worship except Allah, and that Muhammad is the Messenger of Allah, and they establish the prayer, and they give alms. If they do all that then they have protected their blood and wealth from me except by what is due from it, and they will be held to account by Allah the Almighty.”” [163] Reported by al-Bukhaaree (25) and Muslim (22)

Aboo Hurayrah (may Allah be pleased with him) said, “When the Messenger of Allah ?  passed away, and Aboo Bakr (may Allah be pleased with him) was the leader of Muslims, and there were those from the Arabs who disbelieved, ‘Umar (may Allah be pleased with him) said, “How can you fight against people when the Messenger of Allah ? said, “I have been commanded to fight the people until they bear witness that there is no deity worthy of worship except Allah, so whoever says it then they have protected their blood and wealth from me except by what is due from it, and they will be held to account by Allah the Almighty.”?” So, he responded, “By Allah, I will fight whoever makes a distinction between the prayer and zakat, for zakat is the right of wealthy. And if they were to refuse me a young, she-goat that they used to give to the Messenger of Allah ? then I would fight them for their refusal of it. ‘Umar – may Allah be pleased with him – said, “By Allah I did not view it except that Allah had opened Aboo Bakr’s heart towards fighting them, so then I knew it was the truth.”” [164] Reported by al-Bukhaaree – separately in different hadeeths – (1399, 1400, 1456, 1457), and by Muslim (20) and the text is his.

However, if the one who refuses to give the obligatory alms is an individual, then it is forcibly taken from them by consensus. However, they differed in whether they are excommunicated or not, and it is of two opinions. The more correct of the two, is that they are not excommunicated if their abstaining is out of being miserly and not out of denial of it, for there are established explicit texts regarding them not being disbelievers, and that Allah will punish them on the day of Judgement, then they will see their path to either paradise or hellfire. Aboo Hurayrah (may Allah be pleased with him) said, “The Messenger of Allah ? said, “There is not a person who possesses amassed wealth and does not carry out their zakat, except that they are heated in the fire of hell, turned into plates, then they will be cauterised with them on their sides and forehead until Allah judges between His servants, in a day which is the equivalent of fifty thousand years. Then they will see their path; either towards paradise, or towards the hellfire.”” [165] Reported by al-Bukhaaree (4565) in summary, and by Muslim (987) and the text is his. Al-Marwazee said, “What we have mentioned indicates that the one who abstains from giving alms is not a disbeliever nor a polytheist, since he has been given hope of entering paradise, due to Allah the Exalted saying, (Indeed Allah does not forgive that He should be associated with, and He forgives other than that for whomever he wishes.) an-Nisaa’ 48.” [166] “Ta’theem Qadr as-Salaah” (2/1015) And al-Haytamee relates the consensus that abstaining from giving the obligatory alms is a major sin. [167] “Az-Zawaajir ‘an Iqtiraf al-Kaba’ir” (1/287)

Fasting The Month of Ramadan
The fourth pillar from the pillars of Islam is fasting. Allah, Most High, says: O’ you who believe, fasting has been decreed upon you as it has been decreed upon those before you so that you may become pious al-Baqarah: 183.

Ash-Shawkaanee said, “There is no disagreement between the Muslims altogether that the fast of Ramadan is an obligation that Allah, Glorified be He, has obligated upon this Ummah.” [168] “Fath al-Qadir” (1/207)

Ibn Baaz said, “The ruling of the one who abandons the fasting of Ramadan while they are from those who are legally responsible from men and women, is that they have disobeyed Allah and His Messenger and have come with a major sin, and upon them is to repent to Allah from that, as well as make up each fast they abandoned along with feeding the poor for each day if they are capable of doing so. And if they are poor and incapable of feeding others then it is sufficient for them to make up the fasts and to repent. This is because the fast of Ramadan is a great obligation that Allah has decreed upon legally responsible Muslims, and the Prophet ? reported that it is one of the five pillars of Islam. And what is obligated is their being punished for that and their disciplining with what will deter them from that, if their affair is raised to the governor of the people… this is if they do not deny the obligation of fasting Ramadan. As for if they denied the obligation of fasting Ramadan then by that they would be a disbeliever, one who denies that which Allah and His Messenger ? came with. They would be told to repent by the person in authority (i.e., the ruler or whoever he delegates) through the means of legal courts, then they repent. If they do not then their execution is obligated for their apostasy, due to the saying of the Prophet ?, “Whoever changes their religion, then execute him.” [169] Reported by al-Bukhaaree (3017) from the hadeeth of ‘Abdullah ibn ‘Abbas, may Allah be pleased with them both ” [170] “Majmoo’ Fataawa Ibn Baaz” (15/332)

Some of the scholars view that whoever leaves one of the pillars of Islam, such as the fast, then they are to be excommunicated. However, the preponderant position according to the people of knowledge and those who give fatwa, is that the one who abandons fasting is not to be excommunicated. Muhammad ibn Nasr al-Marwazee said, “The people of fatwa and the scholars of the regions are in agreement that whoever intentionally breaks their fast during the month of Ramadan does not get excommunicated because of that, but there is a difference of opinion in what is obligated upon them for that… If they had intentionally broken the fast for the entirety of Ramadan, then some of the scholars obligated upon them that they would have to give an expiation for each day along with making up the fasts… and not a single one from the scholars said that they would have disbelieved.” [171] “Ta’theem Qadr as-Salaah” (2/1016) Al-Haytamee added, “From the major sins is forsaking the fast of a day from the days of Ramadan, and breaking the fast in it by way of sexual relations or otherwise without an excuse.” [172] “Az-Zawaajir ‘an Iqtiraf al-Kaba’ir” (1/323)

Pilgrimage (Hajj)
The fifth pillar from the pillars of Islam, is the pilgrimage for whoever is capable of making the journey.
Allah, Most High, says: Pilgrimage to the House is obliged by Allah upon whoever is capable of doing so. And whoever disbelieves then indeed Allah is not in need of anything from the worlds. aal-‘Imraan :97. Ibn Jareer said, “What He, His Highly Praised be He, means when He, the Exalted, says (And whoever disbelieves then indeed Allah is not in need of anything from the worlds.) is that whoever denies what Allah has obligated from the obligation of pilgrimage to His house, thus rejecting it and disbelieving in it, then Allah is not in need of them, nor of their pilgrimage nor actions, nor of any of His creation from the Jinn and mankind.” [173] “Tafseer Ibn Jareer” (5/618)

Al-Haytamee counted forsaking the pilgrimage while being capable of doing it as being from the major sins. [174] “Az-Zawaajir ‘an Iqtiraf al-Kaba’ir” (1/330) Ibn Baaz said, “To make one pilgrimage during a lifetime is an obligation upon every legally responsible Muslim that is capable of it, from men and women, due to the saying of Allah Almighty, (Pilgrimage to the House is obliged by Allah upon whoever is capable of doing so.) Aal- ‘Imraan: 97 … so the one who forsakes the pilgrimage while being capable of it has disobeyed. However, they are not a disbeliever, rather they are a Muslim; they are prayed over in their funeral, and they are to be buried in the graveyards of the Muslims.” [175] “Fataawa Nurun ‘ala ad-Darb” (17/8)

The ruling of the one who forsakes the four pillars
The people of knowledge have a difference of opinion regarding the excommunication of the one who forsakes the four pillars besides the Two Testimonies of Islam - or some of them – while acknowledging their obligation. Ibn Taymiyyah said, “The Muslims are in agreement that whoever does not come with the Two Testimonies of Islam then they are a disbeliever. As for the four actions then they are in disagreement about the excommunication of the one who forsakes them. And if we say, “The people of Sunnah are in agreement that there is no excommunication for sins” then what we mean by that is acts of disobedience, such as fornication and drinking alcohol. As for these foundational actions then with regards to excommunication over them there is a well-known disagreement.” [176] “Majmoo’ al-Fataawa” (7/302)
Ibn ‘‘Uthaymeen said, “What is correct is that they are not excommunicated except for their forsaking the Two Testimonies of Islam and the prayer. And though it has been related from Imam Ahmad that one is excommunicated for forsaking any of the pillars of Islam, the opposite view is correct.” [177] “Majmoo’ Fataawa wa Rasa’il al- ‘‘Uthaymeen” (9/78) And the intention behind mentioning this issue is to clarify that the opinion of the righteous predecessors regarding the issue of forsaking the genus of actions as a whole is different from their opinion regarding the issue of forsaking the pillars. As for the first one then it is an issue that none of them have differed in, as it is required by their consensus regarding the reality of Imaan, and that it is speech and action. As for the second then that is from the matters of ijtihad (jurisprudential interpretation), even if there is a relationship between the two matters.