Sahih Muslim: Hadith 1769

The Messenger of Allah (ﷺ) 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 (ﷺ) died, this was the practice, and it continued thus during Abu Bakr's caliphate and the early part of 'Umar's caliphate.