Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 125
Narrated 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."