Musnad Abu Ya'la al-Mawsili 7:32

Nasr ibn Ali narrated to us, my father informed me, my father narrated to me, from al-Nadr ibn Shayban, who said: I said to Abu Salamah ibn Abd al-Rahman, "Will you not narrate to us a hadith you heard from your father, which your father heard from the Messenger of Allah, may Allah bless him and grant him peace?" He said: Abd al-Rahman ibn Awf said: "The Messenger of Allah, may Allah bless him and grant him peace, mentioned Ramadan and said: 'Indeed, Ramadan is a month for which Allah has made fasting obligatory, and I have established its standing (in prayer) for the Muslims. So whoever fasts it and stands (in prayer) during it, he will emerge from sin like the day his mother gave birth to him.'"