Sahih Bukhari Volume 3, Book 31, Hadith Number 125.

Narated By Abu Huraira : The Prophet 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.”