Musnad Ahmad 10:33

Surayj ibn al-Nu‘man narrated to us, Nuh ibn Qays narrated to us, from Nasr ibn ‘Ali al-Jahdami, from al-Nadr ibn Shayban al-Haddani, from Abu Salamah ibn ‘Abd al-Rahman, who said: I said to him: "Will you not narrate to me a hadith from your father, which your father heard from the Messenger of Allah, may Allah bless him and grant him peace?" He said: "Ramadan approached, and the Messenger of Allah, may Allah bless him and grant him peace, said: 'Indeed, Ramadan is a month for which Allah, the Almighty and Majestic, has made fasting obligatory, and I have established its standing [in prayer] as a Sunnah for the Muslims. So whoever fasts it out of faith and seeking reward, he will emerge from his sins like the day his mother gave birth to him.'"