Narrated Abbu Huraira (R.A): The Prophet (P.B.U.H) 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.”

(Sahih Bukhari)