It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Whoever stands (in prayer) during Ramadan with faith and seeking reward, all his past sins will be forgiven; and likewise, whoever stands (in prayer) during the Night of Qadr with faith and seeking reward, his past sins will be forgiven.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 10537