Musnad Abu Ya'la al-Mawsili 108:182

Hadba ibn Khalid narrated to us, Aban ibn Yazid narrated to us, Yahya ibn Abi Kathir narrated to us, he said: I asked Abu Salamah, "Which of the Quran was revealed first?" He said, "O you who are wrapped in garments (Al-Muddaththir)." I said, "Indeed, I have been informed that the first surah revealed of the Quran was 'Recite in the name of your Lord who created' (Al-'Alaq)." Abu Salamah said: I asked Jabir ibn Abdullah, "Which of the Quran was revealed first?" He said, "O you who are wrapped in garments (Al-Muddaththir)." I said to him, "Indeed, I have been informed that the first surah revealed of the Quran was 'Recite in the name of your Lord who created' (Al-'Alaq)." Jabir said, "I will not tell you except what the Messenger of Allah, peace and blessings of Allah be upon him, told us. He said: 'I stayed in seclusion in Hira, and when I finished my seclusion, I descended into 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 anything. Then I was called out, and I looked above me, and there he was, sitting on a throne between the heaven and the earth.' He said: 'I fell down from him in fear, then I went to Khadijah and said, "Wrap me up, wrap me up." So they wrapped me up and poured cold water over me. Then there was revealed to me: "O you who are wrapped in garments, arise and warn, and your Lord glorify."'"