From Sahih Bukhari (Hadith 1901):
Narrated Abu Huraira:The Prophet (ﷺ) said, "Whoever established prayers on the night of Qadr out of sincere faith and hoping for a reward from Allah, then all his previous sins will be forgiven; and whoever fasts in the month of Ramadan out of sincere faith, and hoping for a reward from Allah, then all his previous sins will be forgiven
Reference: Bukhari 1901