Believing in the existence of the jinn is obligatory, and denying it constitutes disbelief.
Ibn Battah said: “Whoever denies the existence of the jinn… is a disbeliever in Allah who has rejected His verses and has disbelieved in His Book.” [230] See: al-Sharh wa’l-Ibanah, p. 235.

Ibn Taymiyah said, “The existence of the jinn is proven by the Book of Allah and the Sunnah of His Messenger (blessings and peace of Allah be upon him). The early generations of this ummah and its leading scholars were unanimously agreed on that. The fact that a jinni may enter the body of a human (possession) is also proven according to the consensus of Ahl al-Sunnah wa’l-Jama‘ah.” [231] See: Majmu‘ al-Fatawa, 24/276, 280, 282.

2.1. Textual evidence for the existence of the jinn
2.1.1. Evidence for the existence of the jinn from the Holy Qur’an
i. Allah, may He be glorified and exalted, says (interpretation of the meaning), "Say: It has been revealed to me that a group of the jinn listened to the Qur’an and they said: Indeed, we have heard a wondrous recitation, which guides to the right path, so we have believed in it, and we will never associate anyone with our Lord." (al-Jinn 72:1-2)

ii. Allah, may He be glorified and exalted, says (interpretation of the meaning), "And I did not create the jinn and humans except to worship Me." (al-Dhariyat 51:56)

iii. Allah, may He be glorified and exalted, says (interpretation of the meaning), "And say: My Lord, I seek refuge with You from the goading of the devils. And I seek refuge with You, my Lord, lest they come near me." (al-Mu’minun 23:97-98)

2.1.2. Evidence for the existence of the jinn from the Sunnah
There are many hadiths which refer to the existence of the jinn, including the hadith narrated from ‘A’ishah (may Allah be pleased with her) that the Messenger of Allah (blessings and peace of Allah be upon him) said, “The angels were created from light, the jinn were created from smokeless fire and Adam was created from that which has been described to you.” [232] Narrated by Muslim, 2996.  

2.2. Rational evidence for the existence of the jinn
Not having the tangible proof of a thing does not necessarily mean that it does not exist. In life there are many things that a person is certain exist, yet he does not see them, such as the electricity that runs through wires without him seeing it, or the sound waves that travel through the air, so that he feels their impact without seeing them. [233] See: ‘Abd al-Karim ‘Ubaydat: ‘Alam al-Jinn fi Daw’ al-Kitab wa’l-Sunnah, p. 82; ‘Umar al-Ashqar: ‘Alam al-Jinn wa’l-Shayatin, p. 13.

Muhammad al-Khadr Husayn said, “The one who denies the existence of the jinn is one who thinks only within the framework of tangible things, and does not go even an inch beyond that. We know that we cannot prove their existence on the basis of reason alone, just as we cannot find any evidence to prove that they do not exist. Rather if a person is asked about them and he has sound reasoning and is not influenced by the views of others, he will affirm that it is possible for them to exist, because it is not stipulated that everything that exists should be detectable by one of our five senses. Allah has the power to create beings from very subtle elements, so that they are not visible. So once we accept rationally that it is possible for something to exist, and there is evidence in the religious texts that it does exist, then we should accept what the religious texts tell us, and not differentiate between it and that which we come to know of through seeing it or on the basis of straightforward reasoning and rational evidence.” [234] See: Imam Muhammad al-Khadr Husayn: Mawsu‘at al-A ‘mal al-Kamilah, 2/158.

Abu Bakr al-Jaza’iri said, “The stories and evidence which point to the existence of the jinn and the devils are very many. It is sufficient for us to note the following:

i. Epilepsy, which has been known throughout history in almost every era and country. What we mean by epilepsy is something that is caused by evil spirits, namely the devils… This is the kind of epilepsy which medicine, even in the modern day when it is so advanced, is unable to treat. It is one of the impacts of the jinn and the devils, and offers definitive proof of their existence.

ii. The fact that the jinni may speak on the lips of the human whom he has possessed, and may tell him of things that the afflicted person does not know, to the extent that some of those who are possessed by the jinn may even speak in languages of which the afflicted person does not speak even a single letter! [235] See: ‘Aqidat al-Mu’min, p. 121.