Some details pertinent to the successive stages of Revelation
Before we go into the details of the period of communicating the Message and Prophethood, we would like to get acquainted with the stages of the Revelation which constituted the main source of the Message and the subject-matter of the Call. Ibn Al-Qayyim, mentioning the stages of the Revelation, said:
The First: The period of true vision. It was the starting point of the Revelation to the Messenger of Allâh [pbuh].
The Second: What the angel invisibly cast in the Prophet’s mind and heart. The Messenger of Allâh [pbuh] said: "The Noble Spirit revealed to me ‘No soul will perish until it exhausts its due course, so fear Allâh and gently request Him. Never get so impatient to the verge of disobedience of Allâh. What Allâh has can never be acquired but through obedience to Him.’"
The Third: The angel used to visit the Messenger of Allâh [pbuh] in the form of a human being and would speak to him directly. This would enable him to fully understand what the angel said. The angel was sometimes seen in this form by the Prophet’s Companions.
The Fourth: The angel came to him like the toll of a bell and this was the most difficult form because the angel used to seize him tightly and sweat would stream from his forehead even on the coldest day. If the Prophet [pbuh] was on his camel, the camel would not withstand the weight, so it would immediately kneel down on the ground. Once the Messenger of Allâh [pbuh] had such a revelation when he was sitting and his thigh was on Zaid’s, Zaid felt the pressure had almost injured his thigh.
The Fifth: The Prophet [pbuh] saw the angel in his actual form. The angel would reveal to him what Allâh had ordered him to reveal. This, as mentioned in (Qur’ân), in Sűrah An-Najm (Chapter 53 - The Star), happened twice.
The Sixth: What Allâh Himself revealed to him in heaven i.e. when he ascended to heaven and received Allâh’s behest of Salât (prayer).
The Seventh: Allâh’s Words to His Messenger [pbuh] at first hand without the mediation of an angel. It was a privilege granted to Moses Úáíå ÇáÓáÇã and clearly attested in the Qur’ân, as it is attested to our Prophet [pbuh] in the Sűrah Al-Isrâ’ (Chapter 17 - The Journey by Night) of the Noble Qur’ân.
Some religious scholars added a controversial eighth stage in which they state that Allâh spoke to the Prophet [pbuh] directly without a curtain in between. This issue remains however unconfirmed. [Za'd Al-Ma'ad 1/18]