Musnad Abu Ya'la al-Mawsili 109:226

Abu Khaythama narrated to us, al-Walid narrated to us, he said: Abu ‘Amr al-Awza’i narrated to me, he said: I heard Yahya saying: I asked Abu Salamah, "Which of the Qur’an was revealed first?" He said, "O you who are wrapped in your garment (al-Muddaththir)." I said, "Or 'Read' (Iqra')?" He said: Jabir said, "I shall tell you what the Messenger of Allah, may Allah bless him and grant him peace, told us. He said: 'I stayed in seclusion in Hira for a month. When I completed my seclusion, I descended and reached 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, but I did not see anyone. Then I was called out again, so I looked, but I did not see anyone. Then I raised my head, and there he was, upon a throne in the air'—meaning Gabriel. 'A severe trembling seized me, so I went to Khadija and said, "Wrap me up." So they wrapped me up and poured water over me. Then Allah revealed: "O you who are wrapped in your garment, arise and warn, and your Lord glorify, and your garments purify."'"