Musnad Ahmad 31:534

Ali ibn Ishaq narrated to us, Abdullah—meaning Ibn al-Mubarak—informed us, Yahya ibn Ayyub informed us, from Abdullah ibn Qurayt, that Ata ibn Yasar narrated to him that he heard Abu Sa’id al-Khudri saying: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Whoever fasts Ramadan, and recognizes its limits, and guards himself against what he ought to guard himself against, it expiates what came before it."