Narrated Abu Hurairah (radiyallaahu-anhu): The Prophet (sallal-laahu-alayhi-wasallam) said: ”Whoever established prayer on the night of Qadr with Eemaan and hoping for a reward from Allaah, then all his previous sins will be forgiven.”[Saheeh al-Bukhaari Vol 3: hadeeth Number: 1901]

Imaam Abdul Azeez Bin Baaz (rahimahullaah) said: Having Eemaan that it (i.e. Ramadaan) was legislated by Allaah and seeking reward, as opposed to the one who (fasted) out of blind following. And this forgiveness is restricted with the abandonment of the major sins. (i.e. a person receives this forgiveness when he abandons the major sins). [see Al-Hulalul Ibreeziyyah Min At-taliqaat Al-Baaziyyah Alaa Saheeh Al-Bukhaari: page:120. Vol: 2 Footnote: 6]