Musnad Ahmad 33:175

Al-Walid ibn Muslim narrated to us, Al-Awza’i narrated to us, that he heard Yahya—and Waki’ narrated to us, he said: Ali ibn al-Mubarak narrated to us, from Yahya ibn Abi Kathir, the meaning—he said: I asked Abu Salamah, "Which of the Quran was revealed first?" He said: "O you who are wrapped in garments (Al-Muddaththir)." Yahya said: I said to Abu Salamah, "Or 'Read' (Iqra')?" He said: I asked Jabir, "Which of the Quran was revealed first?" He said: "O you who are wrapped in garments." I said: "Or 'Read'?" Jabir said: I will tell you what the Messenger of Allah, peace and blessings of Allah be upon him, told us. He said: "I stayed in seclusion at 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, so I looked, but I did not see anyone. Then I was called out." Al-Walid said in his narration: "So I raised my head, and behold, he was upon a throne in the air, and a severe trembling seized me." And they both said in their narration: "So I came to Khadijah and said: 'Wrap me up, wrap me up,' and they wrapped me up and poured water over me. Then Allah revealed: 'O you who are wrapped in garments, arise and warn, and your Lord magnify, and your garments purify.'"