Sahih Bukhari Volume 3, Book 32, Hadith Number 231.

Narated By Abu Huraira : The Prophet 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.”