Musnad Ahmad 30:162

Sufyan narrated to us, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him): "Whoever fasts Ramadan out of faith and seeking reward, his past sins will be forgiven." Abdullah ibn Ahmad said: My father said: "I heard it four times from Sufyan." And he said once: "Whoever fasts Ramadan," and he said once: "Whoever stands [in prayer]." "And whoever stands [in prayer] on the Night of Decree out of faith and seeking reward, his past sins will be forgiven."