Sahih Muslim Book 4, Hadith Number 1575.

Chapter : Excellence of the forenoon prayer, two are its minimum Rak’ahs and eight are its maximum Rak’ahs and average Rak’ahs are four or six, and exhortation for the observance of the prayer.

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.).