Sahih Bukhari Volume 1, Book 2, Hadith Number 36

Narated By Abu Huraira : Allah’s Apostle 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.”

References

Reference: Sahih al-Bukhari 37
In-Book Reference: Sahih al-Bukhari Book 2 Hadith 30