Sahih Muslim 161d

Zuhayr ibn Harb narrated to us: al-Walid ibn Muslim narrated to us; al-Awza'i narrated to us. He said: I heard Yahya say: I asked Abu Salama which part of the Qur'an was revealed first. He said: O you who are wrapped up. I said: Or Read? He said: I asked Jabir ibn 'Abdullah which part of the Qur'an was revealed first. He said: O you who are wrapped up. I said: Or Read? Jabir said: I will relate to you what the Messenger of Allah (peace and blessings be upon him) related to us. He said: I stayed in retreat at Hira for a month. When my retreat was over, I came down and went deep into the valley. I heard a call and looked ahead, behind me, to my right, and to my left, but saw no one. I was called again; I looked but saw no one. I was called a third time, so I raised my head and saw him—Gabriel, peace be upon him—seated on a throne in the open air. I began to tremble violently and went to Khadija, saying: Cover me. They covered me and poured water over me. Then Allah, Mighty and Majestic, revealed: O you who are wrapped up! Arise and warn; magnify your Lord; and purify your garments.