Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 446
Narrated 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).