Sahih Muslim Book 4, Hadith Number 1665.

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 Apostle (may peace be upon him) as saying: He who prayed on the Lailat-ul-Qadr (the Majestic Night) knowing that it is (the same night). I (believe) that he (the Holy Prophet also) said: (He who does) it with faith and seeking reward (from Allah), his sins would be forgiven.