Abu Dawud Book 2, Hadith Number 1366.
Chapter : Night prayers during ramadhan.
Narated By Abu Hurairah : The Apostle of Allah (PBUH) used to commend prayers at night during Ramadhan, but did not command it as a duty. He would said: If anyone prays during the night in Ramadhan because of faith and seeking his reward from Allah, his previous sins would be forgiven to him. When the Apostle of Allah (PBUH) died, this was the practice, and it continued thus during the Abu Bakr’s caliphate and early part of ‘Umars.