Musnad Abu Ya'la al-Mawsili 155:90

Abu Bakr ibn Abi Shaybah narrated to us, Ibn Fudayl narrated to us, from Yahya ibn Sa'id, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever fasts Ramadan out of faith and seeking reward, his past sins will be forgiven."