Wahy (revelation) may come in the form of inspiration (ilham). That is like the inspiration that Allah sent to the disciples, "And remember when I inspired the disciples to believe in Me and in My Messenger." (al-Ma’idah 5:111) And it is like the inspiration that Allah sent to the mother of Musa (peace be upon him), "We inspired the mother of Musa, saying: Breastfeed him." (al-Qasas 28:7) Ibn Taymiyah said, “These people were not prophets; rather that was inspiration (ilham).”

Wahy may also refer to gesturing and signaling. The Qur’an calls the signaling of Zakariyya to his people “wahy,” as Allah, may He be glorified and exalted, tells us of him (interpretation of the meaning), "So he came out to his people from the chamber, and signaled awha to them to glorify Allah in the morning and in the afternoon." (Maryam 19:11)

Al-Shinqiti said, “Regarding the signaling wahy mentioned in the verse and signaled to them to glorify Allah, some of the scholars said that this refers to gesturing, and this is most likely to be the case.”

In most cases, this word (wahy) is used in the Qur’an to refer to Allah telling and informing those whom He chose from among His slaves everything that He wanted to tell them about of different types of guidance and knowledge in a subtle and hidden way that was not the norm for people.

‘Abd al-Razzaq ‘Afifi said: “In linguistic terms, wahy means to inform in a subtle manner, by gestures, writing, inspiration, conversing, and so on.

In Islamic terminology, it refers to Allah informing His Prophet of a shar‘i ruling and the like, through an intermediary or otherwise.” There are various levels and ways in which the message that Allah conveys to His prophets and messengers reaches them. Allah, may He be glorified and exalted, says, explaining that (interpretation of the meaning), "It is not fitting for any man that Allah should speak to him except by revelation or from behind a veil, or by sending a Messenger to reveal, by Allah’s leave, whatever Allah wills. Verily, He is Most High, Most Wise." (al-Shura 42:51)

Ibn Jarir said, “Here Allah, may He be glorified and exalted, says that it is not appropriate for any man among the descendants of Adam that Allah should speak to him except by way of revelation which Allah reveals to him in whatever manner He wills, either through divine inspiration or otherwise; or from behind a veil that is, or He may speak to him so that he hears His words without seeing Him, as He spoke to His Prophet Musa (blessings and peace of Allah be upon him); or by sending a Messenger that is, or Allah sends one of His angels as a messenger, either Jibril or someone else; to reveal, by Allah’s leave, whatever Allah wills that is, and that angelic messenger reveals to the one to whom he is sent, by his Lord’s leave, whatever He wills – in other words, whatever it is his Lord’s will to reveal to him of commands, prohibitions or other matters of the divine message and revelation.”

i. Revelation from Allah, may He be glorified and exalted
Al-Baghawi said: It is not fitting for any man that Allah should speak to him except by revelation – He may send revelation to him in a dream or through inspiration.” It was narrated that ‘A’ishah (may Allah be pleased with her) said, “The first thing with which the revelation began for the Messenger of Allah (blessings and peace of Allah be upon him) was true dreams which he saw in his sleep; he did not see any dream but it came true like the light of dawn.” 

‘Iyad said, “This indicates that true dreams are one of the characteristics of prophethood, a sign of honor and a part of prophethood. It is the first stage of revelation, and the dreams of the prophets are revelation and are true.”

ii. Allah speaking to His prophets and messengers directly from behind a veil
This is as when Allah, may He be glorified and exalted, spoke directly to Musa (peace be upon him). Allah, may He be glorified and exalted, says (interpretation of the meaning), "And when Musa arrived at Our appointed time and his Lord spoke to him." (al-A‘raf 7:143) Al-Shawkani said, “and his Lord spoke to him means that He made him hear His words without any intermediary.” Al-Halimi said, “The Prophets (peace be upon them) were singled out exclusively, beyond what I have described, and were confirmed by means of signs so that they would be distinguished from those who were not like them, just as they were distinguished by the knowledge they were given… And the manner in which they received this knowledge varied. The highest level of that was when Allah, may He be glorified and exalted, spoke directly to some of them.”

iii. Allah informing His prophet or messenger through an angel
Ibn Juzay said about Allah's verse, “It is not fitting for any man that Allah should speak to him except by revelation." (al-Shura 42:51), here Allah, may He be glorified and exalted, explains how He speaks to His slaves, and stated that He speaks to them in three ways…

The third way is revelation through the angel, as He says, "or by sending a Messenger that is, an angel to reveal, by Allah’s leave." Whatever Allah wills to the prophet. This is only for the prophets.” There are three ways in which the angel came to the Prophet (blessings and peace of Allah be upon him):

a. The first way was when the Messenger (blessings and peace of Allah be upon him) saw the angel in the form in which Allah had created him. This only happened to Muhammad (blessings and peace of Allah be upon him) twice. Allah, may He be glorified and exalted, says (interpretation of the meaning), "It is not but a revelation revealed. Taught to him by one intense in strength. One of soundness. And he rose to his true form. When he was above the horizon. Then he approached and descended. And was at a distance of two bow lengths or nearer. Then he revealed to Allah’s slave what he had to reveal. His heart did not deny what he saw. So will you dispute with him over what he saw? And he certainly saw him in another descent. At the Lote Tree of the Utmost Boundary. Near it is the Garden of Refuge." (al-Najm 53:4-15) Ibn Kathir said, “This seeing of Jibril did not happen on the night of the Isra’ (night journey); rather it happened before that, when the Messenger of Allah (blessings and peace of Allah be upon him) was on the ground, and Jibril (peace be upon him) came down to him from above, then he approached him in the form in which Allah created him, with six hundred wings. Then he saw him after that at a second descent at the Lote Tree of the Utmost Boundary, that is, on the night of the Isra’. This first seeing occurred at the beginning of his mission.”

b. The revelation came to him like the ringing of a bell, then no sooner had it left him but the Messenger (blessings and peace of Allah be upon him) understood what the angel had said.

c. The angel appeared to him in the form of a man who spoke to him and addressed him, and he understood what he said. ‘A’ishah the Mother of the Believers (may Allah be pleased with her) narrated that al-Harith ibn Hisham (may Allah be pleased with him) asked the Messenger of Allah (blessings and peace of Allah be upon him), "O Messenger of Allah, how does the revelation come to you? The Messenger of Allah (blessings and peace of Allah be upon him) said: “Sometimes it comes like the ringing of a bell, and that is the hardest for me, but as soon as it is over I have understood it. Sometimes the angel comes in the form of a man who speaks to me, and I understand what he says.” (al-Bukhari, Muslim)

Al-Nawawi said, “The scholars said: in this hadith, the Prophet (blessings and peace of Allah be upon him) mentioned two of the ways in which the revelation came to him: like the ringing of a bell, and when the angel appeared in the form of a man. He did not mention dreams, which are also part of the revelation, because the aim of the questioner was to know what was unique to the Prophet (blessings and peace of Allah be upon him), as it was not clear.”

The third category may also include what is mentioned in the verse in which Allah, may He be glorified and exalted, says (interpretation of meaning), "Or by sending a Messenger to reveal, by Allah’s leave, whatever Allah wills. Verily, He is Most High, Most Wise." (al-Shura 42:51) This refers to Jibril casting the message into the heart of the Prophet (blessings and peace of Allah be upon him) without him seeing Jibril.

Abu Umamah (may Allah be pleased with him) narrated that the Messenger of Allah (blessings and peace of Allah be upon him) said, “The Pure Spirit cast into my heart that no soul will die until its lifespan is complete and it has received its provision in full, so be moderate in seeking your provision.” (Da ‘if) Al-Halimi said, “Another way is that Allah instructed the angels to cast into his heart… This is revelation that is only for the heart and is not heard.” 

There were harbingers and signs of the revelation before it happened. Ibn ‘Abbas (may Allah be pleased with him) said, "The Messenger of Allah (blessings and peace of Allah be upon him) stayed in Makkah for fifteen years, hearing the voice and seeing the light (i.e., of angels); seven years when he did not see any visible form and eight years when he received revelation, and he stayed in Madinah for ten years." (Muslim)

Abu’l-‘Abbas al-Qurtubi said, “That is, the voices of angels, inanimate objects and rocks, which greeted him as a messenger… What is meant by light is the light of the angels, or it may be that they were lights that shone ahead of him at times of darkness, that others could not see… Then after that, Allah sent revelation to him.”