From Sahih Bukhari (Hadith 2009):
Narrated Abu Huraira: Allah's Messenger (ﷺ) said, "Whoever prayed at night the whole month of Ramadan out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven." Ibn Shihab (a sub-narrator) said, "Allah's Messenger (ﷺ) died and the people continued observing that (i.e. Nawafil offered individually, not in congregation), and it remained as it was during the Caliphate of Abu Bakr and in the early days of 'Umar's Caliphate
Reference: Bukhari 2009