From Sahih Bukhari (Hadith 2014):
Narrated Abu Huraira:The Prophet (ﷺ) said, "Whoever fasted the month of Ramadan out of sincere Faith (i.e. belief) and hoping for a reward from Allah, then all his past sins will be forgiven, and whoever stood for the prayers in the night of Qadr out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven
Reference: Bukhari 2014