Book 001, Number 0307:

Yahya reported: I asked Abu Salama what was revealed first from the Qur'an. He said:" 0, the shrouded one." I said: Or" Recite." Jabir said: I am narrating to you what was narrated to us by the Messenger of Allah (may peace be upon him). He said: I stayed in Hira' for one month and when my stay was completed, I come down and went into the heart of the valley. Somebody called me aloud. I looked in front of me, behind me, on the right of my side and on my left, but I did not see any body. I was again called and I looked about but saw nothing. I was called again and raised my head, and there on the Throne in the open atmosphere he, i. e. Gabriel (peace be upon him) was sitting. I began to tremble on account of fear. I came to Khadija and said: Wrap me up. They wrapped me up and threw water on me and Allah, the Exalted and Glorious, sent down this: you who are shrouded! arise and deliver warning, your Lord magnify, your clothes cleanse."