Abu Dawud Book 2, Hadith Number 1251.
Chapter : On observing the sunan prayer at dawn lightly.
Narated By Abu Hurairah : The Prophet (PBUH) would recite in both the rak’ahs of the dawn prayer: “Say, O unbelievers” (Surah cix) and “Say: he is Allah, the One” (Surah cxii.).