Musnad Abu Ya'la al-Mawsili 42:85

Abdullah ibn Umar ibn Aban narrated to us, Abdullah ibn al-Mubarak narrated to us, from Yahya ibn Ayyub, from Abdullah ibn Qurayt, from Ata ibn Yasar, from Abu Sa’id al-Khudri, from the Prophet, may Allah bless him and grant him peace, who said: "Whoever fasts Ramadan, and recognizes its limits, and guards what is necessary for him to guard from it, it will expiate what came before it."