Musnad Ahmad 30:1880

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, from Muhammad ibn 'Amr, from Abu Salamah, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said—Hammad and Thabit said, from al-Hasan, from the Prophet (peace and blessings of Allah be upon him)—he said: "Whoever fasts Ramadan out of faith and seeking reward, his past and future sins will be forgiven for him."