Musnad Ahmad 10:6

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Al-Qasim ibn al-Fadl narrated to us, An-Nadr ibn Shayban narrated to us, he said: I met Abu Salamah ibn ‘Abd al-Rahman, and I said: Narrate to me something you heard from your father, which he heard from the Messenger of Allah, may Allah bless him and grant him peace, regarding the month of Ramadan. He said: Yes, my father narrated to me from the Messenger of Allah, may Allah bless him and grant him peace, that he said: "Indeed, Allah the Almighty has obligated the fasting of Ramadan, and I have established its standing (in prayer). So whoever fasts it and stands (in prayer) during it, out of faith and seeking reward, will emerge from his sins like the day his mother gave birth to him."