Musnad Ahmad 30:3406

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts Ramadan and stands in prayer during it, out of faith and seeking reward, his past sins will be forgiven. And whoever stands in prayer on the Night of Decree (Laylat al-Qadr), out of faith and seeking reward, his past sins will be forgiven."