Musnad Abu Ya'la al-Mawsili 108:181

Abu Abdullah Ahmad ibn Ibrahim al-Nukri narrated to us, Mubashir narrated to us, from al-Awza’i, from Yahya ibn Abi Kathir, who said: I asked Abu Salamah ibn Abd al-Rahman, "Which of the Quran was revealed first?" He said, "O you who are wrapped in your garments (al-Muddaththir)." I said, "Or 'Read in the name of your Lord who created' (al-‘Alaq)?" He said, "I asked Jabir ibn Abdullah, 'Which of the Quran was revealed first?' He said, 'O you who are wrapped in your garments.' I said, 'Or "Read in the name of your Lord who created"?'" Jabir said, "I will not tell you except what the Messenger of Allah told us. The Messenger of Allah (peace and blessings of Allah be upon him) said: 'I stayed in Hira for a month. When I finished my stay, I descended and went into the middle of the valley. I was called out, so I looked in front of me, behind me, to my right, and to my left, but I did not see anyone. Then I was called out again, so I looked in front of me, behind me, to my right, and to my left, but I did not see anyone. Then I looked toward the sky, and there he was, upon a throne in the air.'" Mubashir said: Meaning Gabriel. "So I came and went to Khadijah and said, 'Cover me, cover me.' So they covered me, and Allah revealed: 'O you who are wrapped in your garments, arise and warn, and your Lord glorify, and your garments purify.'"