Sahih Bukhari Volume 1, Book 2, Hadith Number 37

Narated By Abu Huraira : Allah’s Apostle said, “Whoever observes fasts during the month of Ramadan out of sincere faith, and hoping to attain Allah’s rewards, then all his past sins will be forgiven.”

References

Reference: Sahih al-Bukhari 38
In-Book Reference: Sahih al-Bukhari Book 2 Hadith 31