Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him) said: Whoever fasts during Ramadan with faith and seeking reward, his previous sins will be forgiven; and whoever stands (in prayer) during the Night of Qadr with faith and seeking reward, his previous sins will be forgiven.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصيام / 404