Abu Hurairah (radi-Allaahu ‘anhu) reported that the Messenger of Allaah (sal-Allaahu ‘alayhe wa sallam) said:
“Whoever performs the night prayer on the night of Al-Qadr with Eemaan (firm belief) and seeking reward will have all his past sins forgiven.” – Al-Bukhaaree (4/2550) and Muslim (759).
This hadeeth is evidence for the virtue of the night of Al-Qadr as well as performing qiyaam (night prayer) during it. And it indicates that it is a grand night, which Allaah has honored and made better than a thousand months, with regard to its blessing and the blessings found in the righteous deeds that are performed in it. Thus it is better than the worship of a thousand months and that is equivalent to eighty-three years and four months. Due to this, whoever performs qiyaam (night prayer) with true faith and while seeking reward in it, will be forgiven his past sins.
Taken from Ahadeeth As- Siyaam: Ahkaam wal Adaab (pg. 141-143) by Shaykh ‘Abdillah ibn Saalih Al- Fawzaan