Sahih Bukhari Volume 6, Book 65, Hadith 4926

حَدَّثَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، حَدَّثَنَا عَبْدُ الصَّمَدِ، حَدَّثَنَا حَرْبٌ، حَدَّثَنَا يَحْيَى، قَالَ سَأَلْتُ أَبَا سَلَمَةَ أَىُّ الْقُرْآنِ أُنْزِلَ أَوَّلُ فَقَالَ ‏{‏يَا أَيُّهَا الْمُدَّثِّرُ‏}‏ فَقُلْتُ أُنْبِئْتُ أَنَّهُ ‏{‏اقْرَأْ بِاسْمِ رَبِّكَ الَّذِي خَلَقَ‏}‏ فَقَالَ أَبُو سَلَمَةَ سَأَلْتُ جَابِرَ بْنَ عَبْدِ اللَّهِ أَىُّ الْقُرْآنِ أُنْزِلَ أَوَّلُ فَقَالَ ‏{‏يَا أَيُّهَا الْمُدَّثِّرُ‏}‏ فَقُلْتُ أُنْبِئْتُ أَنَّهُ ‏{‏اقْرَأْ بِاسْمِ رَبِّكَ‏}‏ فَقَالَ لاَ أُخْبِرُكَ إِلاَّ بِمَا قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ جَاوَرْتُ فِي حِرَاءٍ فَلَمَّا قَضَيْتُ جِوَارِي، هَبَطْتُ فَاسْتَبْطَنْتُ الْوَادِيَ فَنُودِيتُ، فَنَظَرْتُ أَمَامِي وَخَلْفِي وَعَنْ يَمِينِي وَعَنْ شِمَالِي فَإِذَا هُوَ جَالِسٌ عَلَى عَرْشٍ بَيْنَ السَّمَاءِ وَالأَرْضِ، فَأَتَيْتُ خَدِيجَةَ فَقُلْتُ دَثِّرُونِي وَصُبُّوا عَلَىَّ مَاءً بَارِدًا، وَأُنْزِلَ عَلَىَّ ‏{‏يَا أَيُّهَا الْمُدَّثِّرُ * قُمْ فَأَنْذِرْ * وَرَبَّكَ فَكَبِّرْ‏}‏‏"‏

Narrated Yahya: I asked Aba Salama, "Which Surah of the Quran was revealed first?" He replied,"O you(Muhammad(s.a.w), enveloped (in garments)! (Al-Muddaththir No. 74)." I said, "I have been informed that it was , 'Read! In the name of your Lord! who has created...' (Surat Al-Alaq No. 96)." Abu Salama said, "I asked Jabir, 'Which Sura of the Quran was revealed first?'. He said, "O you(Muhammad(s.a.w)) enveloped (in garments)!" I said, "I have been told that it was 'Read! In the name of your Lord, who has created. "He said, 'I will not tell you but what Allah's Messenger (s.a.w) said.' Allah's Messenger(s.a.w) said, 'I was in seclusion in the cave of Hira' ' and when I completed the limited period of my seclusion, I came down till I reached the valley. I heard a voice calling me, so I looked in front of me, behind me, to my right, to my left, and Behold! I saw (an angel) sitting on a throne between the sky and the earth. So, I went to Khadija and told her to envelop me in garments and pour cold water on me. Then, it was revealed to me: 'O you (Muhammad(s.a.w)) enveloped(in garments)! Arise and warn! And magnify your Lord(Allah)!' "(V.74:1-3)