“مَنْ صَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " رواه البخاري
"whoever fasts during Ramadan out of sincere faith & hoping to attain Allah’s rewards, then all his past sins will be forgiven.”
(Al-Bukhari)