Musnad Abu Ya'la al-Mawsili 155:157

And from Abu Hurairah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever fasts Ramadan out of faith and seeking reward, his past sins will be forgiven. And whoever stands (in prayer) on the Night of Decree out of faith and seeking reward, his past sins will be forgiven."