Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1663
Abu Huraira reported: The Messenger of Allah (may peace be upon him) used to exhort (his Companions) to pray (at night) during Ramadan without commanding them to observe it as an obligatory act, and say: He who observed the night prayer in Ramadan because of faith and seeking his reward (from Allah), all his previous sins would be forgiven. When Allah's Messenger (may peace be upon him) died, this was the practice, and it continued thus during Abu Bakr's caliphate and the early part of 'Umar's caliphate.