Narrated Abu Huraira (R.A)
The Prophet Sallallahu Alaihi
Wasallam 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, Book 31, No. 125}