Narrated Abu Huraira (R.A.A.): Allah's Apostle (S.A.W.) said: "Whoever establishes prayers during the nights of Ramadan faithfully out of sincere faith and hoping to attain Allah's rewards (not for showing off), all his past sins will be forgiven."
(Bukhari, Volume 1, Book 2, Number 36)
Commentery: The word “sins” applies to minor sins only because major ones will not be forgiven without sincere repentance.