"....then all his past sins will be forgiven."

Sahīh Bukhari (1/38)

Narrated Abu Hurairah [رضي الله عنه]: Allāh's Messenger ﷺ said:

❝Whoever observes Saum [fasts] during the month of Ramadān out of sincere faith, and hoping to attain Allāh's Rewards, then all his past sins will be forgiven.❞

[Sahīh Bukhari, (1/38) | Translated By Muhsin Khan]