Sahih Muslim Book 4, Hadith Number 1664.

Chapter : Exhortation to supplicate and make mention (of Allah) at the end of the night and its acceptance by the Lord.

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.