Wahy (Divine Revelation) is the inspiration and guidance given by Allah Almighty to his Prophets. Wahy came to Prophet Muhammad (Sal Allahu Alayhi Wa Sallam) in several modes.
1. Chiming of Bells
Sayyidah Aishah (Radhi Allahu Anha) says that Sayyidna Harith ibn Hisham (Radhi Allahu Anhu) once asked the Holy Prophet (Sal Allahu Alayhi Wa Sallam) how Wahy came to him. The Holy Prophet (Sal Allahu Alayhi Wa Sallam) said “There are times when I hear something like the chiming of bells and this mode of Wahy is the hardest on me. After the chiming ends, the revelation is committed to my memory. And there are times when the angel appears before me in the shape of a man and talks to me.”
Sayyidah Aishah (Radhi Allahu Anha) adds, ”I saw the Prophet (Sal Allahu Alayhi Wa Sallam) being inspired with Wahy on a very cold day and when the Wahy ceased, sweat would be dripping from his forehead in spite of the cold weather.” (Sahih al-Bukhari, 2/1)
In another narration, Sayyidah Aishah (Radhi Allahu Anha) relates, “When Wahy came to the Prophet (Sal Allahu Alayhi Wa Sallam), his breath would seem to stop, the radiant face would turn pale like the branch of a date palm, the front teeth would shiver from cold and he would perspire so much that its drops would roll like pearls.” (Al-Itqan, 1/46)
On one occasion the camel that the Prophet (Sal Allahu Alayhi Wa Sallam) was riding at the time of revelation knelt down unable to bear the weight.
Once, when the Prophet (Sal Allahu Alayhi Wa Sallam) was resting his head on the lap of Sayyidna Zayd ibn Thabit (Radhi Allahu Anhu), the descent of Wahy started and this put so much weight on Sayyidna Zayd’s thigh that Sayyidna Zayd felt it would break. (Zad al-Ma’ad, 1/18, 19)
There were times when a low-volumed sound of this Wahy was sensed by others as well. Sayyidna Umar (Radhi Allahu Anhu) says, “When Wahy came to the Prophet (Sal Allahu Alayhi Wa Sallam), a sound similar to the buzzing of honey-bees could be heard close to the Prophet’s (Sal Allahu Alayhi Wa Sallam) most bright face.” (Tabwib Musnad Ahmad, Kitab al-Sirah al-Nabaviyah, 20/212)
2. Jibrail (Alayhis Salam) in Human Form
In this mode of Wahy, Sayyidna Jibrail (Alayhis Salam) would come to Prophet Muhammad (Sal Allahu Alayhi Wa Sallam) in human form and deliver Allah’s message. Sayyidna Jibrail (Alayhis Salam) usually came in the form of Sayyidna Dihyah al-Kalbi (Radhi Allahu Anhu), a companion of Prophet Muhammad (Sal Allahu Alayhi Wa Sallam). Sayyidna Jibrail (Alayhis Salam) also came in other human forms. This mode of Wahy was the easiest on the Holy Prophet (Sal Allahu Alayhi Wa Sallam). (Al-Itqan, 1/46)
3. Jibrail (Alayhis Salam) in Angelic Form
In this mode of Wahy, Sayyidna Jibrail (Alayhis Salam) would appear in his angelic form. This occured only thrice. First, when the Holy Prophet (Sal Allahu Alayhi Wa Sallam) wished to see Jibrail (Alayhis Salam) in his real form. Second, during Miraj (the Ascent to Heaven) and third at Ajyad in Makkah al-Mukarramah during the very early days of prophethood. The first two happenings are authentic, however, the last one suffers from weak chains of authority and is, therefore, doubtful. (Fath al-Bari, 1/18,19)
4. Conversation with Allah Almighty
Another mode of Wahy was direct conversation with Allah Almighty. This honour was bestowed upon the Holy Prophet (Sal Allahu Alayhi Wa Sallam) only once while awake, during Miraj (Ascent to Heaven). Prophet Muhammad (Sal Allahu Alayhi Wa Sallam) also conversed with Allah Almighty in a dream once.
5. Blowing into the Heart
In this mode of Wahy, Sayyidna Jibrail (Alayhis Salam) without appearing physically, would let some words of the message fall into Prophet Muhammad (Sal Allahu Alayhi Wa Sallam)’s heart. This is known as Nafth Fi al-Rau (blowing into the heart). (Ibid)
Reference: Ma’ariful Quran (The Wisdom of the Holy Qur’an) - Volume 1 by Maulana Mufti Muhammad Shafi’