Sahih Bukhari Volume 6, Book 60, Hadith Number 446.

Narrated By Yahya : I asked Aba Salama, “Which Sura of the Qur’an was revealed first?” He replied, “O you, wrapped-up’ (Al-Muddaththir).” I said, “I have been informed that it was, ‘Read, in the Name of your Lord who created… (i.e. Surat Al-Alaq).