Yahya bin Kathir reported,

I asked Abu Salama bin Abdul-Rahman about the first part of the Quran to be revealed, and he said, "You who are wrapped in a cloak" [Muddaththir 74:1].I said, "But people say it was 'Recite in the name of your Lord who created' [Alaq 96:1]."Abu Salama replied, "I asked Jabir bin Abdullah about that and put to him the same thing you have just said to me."Jabir answered, "I will tell you nothing except what the Messenger of Allah ﷺ told us."He said, "I had been staying in seclusion at Hira, and when my period of seclusion was over I came down. Then I was called. I looked to my right and saw nothing, I looked to my left and saw nothing, I looked in front of me and saw nothing, and I looked behind me and saw nothing. Then I raised my head and saw something. So I went to Khadijah and said, 'Wrap me up, and pour cold water over me.' They wrapped me up and poured cold water over me, and then there was revealed, 'You who are wrapped in a cloak, arise and warn, and magnify your Lord' [Muddaththir 74:1–3]."

عن يحيى بن كثير قال

سألت أبا سلمة بن عبد الرحمن عن أول ما نزل من القرآن فقال: {يأيها المدثر} [المدثر: ١] قلت: يقولون: {اقرأ باسم ربك الذي خلق} فقال أبو سلمة: سألت جابر بن عبد الله عن ذلك وقلت له مثل الذي قلت فقال جابر: لا أحدثك إلا ما حدثنا رسول الله ﷺ قال: جاورت بحراء، فلما قضيت جواري هبطت، فنوديت، فنظرت عن يميني فلم أر شيئا، ونظرت عن شمالي فلم أر شيئا، ونظرت أمامي فلم أر شيئا، ونظرت خلفي فلم أر شيئا، فرفعت رأسي فرأيت شيئا، فأتيت خديجة فقلت: دثروني، وصبوا علي ماء باردا قال: فدثروني وصبوا علي ماء باردا، قال: فنزلت: {يأيها المدثر قم فأنذر وربك فكبر} [المدثر: ١- ٣].