Abu Huraira reported:
The Messenger of Allah ( صلی اللہ علیہ وسلم ) 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 ( صلی اللہ علیہ وسلم ) died, this was the practice, and it continued thus during Abu Bakr's caliphate and the early part of 'Umar's caliphate.