Fasting and Praying in Ramadan is Expiation of Sins

Sahih Bukhari
Volume 3, Book 32, Number 231:

Narrated Abu Huraira:
The Prophet said, "Whoever fasted the month of Ramadan out of sincere Faith (i.e. belief) and hoping for a reward from Allah, then all his past sins will be forgiven, and whoever stood for the prayers in the night of Qadr out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven ."