Sahih Bukhari
Praying at Night in Ramadaan (Taraweeh)
Volume: 003 / Book Number: 032 / Hadith Number: 226
Narrated Abu Huraira:
I heard Allah's Apostle saying regarding Ramadan, "Whoever prayed at night in it (the month of Ramadan) out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven."