Musnad Abu Ya'la al-Mawsili 7:30

Shayban ibn Farrukh narrated to us, Qasim ibn al-Fadl narrated to us, al-Nadr ibn Shayban narrated to us, from Abu Salamah ibn Abd al-Rahman, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts Ramadan out of faith and seeking reward, he emerges from his sins like the day his mother gave birth to him."