It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: Whoever fasts during Ramadan with faith and seeking reward, all his past sins will be forgiven. My father says that I heard this hadith from Sufyan four times, and in it there is also this addition: Whoever stands (in prayer) during the Night of Qadr with faith and seeking reward, all his past sins will be forgiven.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 7280