Abu Dawud Book 2, Hadith Number 1367.
Chapter : Night prayers during ramadhan.
Narated By Abu Hurairah : The Prophet (PBUH) said: If anyone fasts during Ramadhan because of faith and and in order to seek his reward from Allah, his previous sins would be forgiven to him. If anyone prays in the night of power(lailat-ul-qadr) because of faith and and in order to seek his reward from Allah, his previous sinswould be forgiven to him.
Abu Dawud said: this tradition has been transmitted in a simmilar manner by Yahya b. Abi Kathir and Muhammad b. ‘Amr from Abu Salamah.