Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1664
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: He who observed the fasts of Ramadan with faith and seeking reward (from Allah), all his previous sins would be forgiven, and he who observed prayer on Lailat-ul- Qadr with faith and seeking reward (from Allah), all his previous sins would be forgiven.