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, "Wasn't Surah Iqra' revealed first?" He replied, "I asked the same question to Sayyiduna Jabir (may Allah be pleased with him), and he gave the same answer. I also asked this question, and he said, 'I am telling you what the Prophet (peace be upon him) himself told us.' The Prophet (peace be upon him) said, 'I remained a neighbor in 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 left, but I saw no one. After a short while, the voice came again. I looked all around again but saw no one. The third time the voice came, I raised my head and saw Jibril (peace be upon him) in the air on his throne. Seeing this, I was overtaken by a severe 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 «يا ايها المدثر قم فانذر» to the end.'"
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 14287