Musnad Ahmad Hadith 15214

Yahya bin Abi Kathir says that I asked Abu Salamah which part of the Qur'an was revealed first. He mentioned Surah Al-Muddathir. I said: Was Surah Iqra’ not revealed first? He said that I had asked this same question to Jabir (may Allah be pleased with him), and he gave this answer, and I also asked this question, so he said: I am telling you what the Prophet (peace be upon him) himself told us. The Prophet (peace be upon him) said: I stayed in the neighborhood of the cave of Hira for a month. When I completed the month and came down from the mountain and reached the bottom of the valley, someone called out to me. I looked ahead, behind, to my right and to my left, but I saw no one. After a short while, the voice came again. I looked around again, but saw no one. The third time the voice came, I raised my head and saw Jibril in the air on his throne. Seeing this, I was seized with intense trembling, and I went to Khadijah and said, "Cover me with a thick blanket." So she covered me with a blanket and poured water over me. At that moment, Allah revealed this verse: "O you who covers himself [with a cloak], arise and warn..." to the end.