Sahih al-Bukhari 4924

It is related that Yahya said, "I asked Abu Salama, 'Which part of the Qur'an was revealed first?' He answered, "O you enveloped in your cloak! (74:1)" I said, 'I have been told that it was "Read, in the Name of your Lord. (96:1)"' Abu Salama said, 'I asked Jabir ibn 'Abdullah, "Which part of the Qur'an was revealed first?" He answered, "O you enveloped in your cloak! (74:1)" I said, "I have been told that it was 'Read, in the Name of your Lord. (96:1)'" He said, "I only report to you what the Messenger of Allah, may Allah bless him and grant him peace, said. The Messenger of Allah , may Allah bless him and grant him peace, said, "I was in retreat in Hira'. When I finished my retreat, I descended. When I reached the bottom of the valley, I was called. I looked in front of me and behind me, and to my right and my left. There he [the angel] was sitting on a throne between the sky and earth. I went to Khadija and said, 'Wrap me up and pour cold water on me!' and then there was revealed to me, 'O you enveloped in your cloak, arise and warn! Magnify your Lord.' (74:1-3)"'"