Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1575
Abu Huraira reported that the Messenger of Allah (may peace be upon him) recited in the two (supererogatory) rak'ahs of the dawn (prayer):" Say: O unbelievers," (Qur'an, cix.) and" Say: Allah is one" (cxii.).