Musnad Abu Ya'la al-Mawsili 1:135

Yahya ibn Abd al-Hamid al-Hummani narrated to us, my father narrated to us, from Uthman ibn Waqid, from Abu Nasirah, who said: I met a freed slave of Abu Bakr and said to him: Did you hear Abu Bakr say that the Messenger of Allah (peace and blessings of Allah be upon him) said: "He has not persisted [in sin] who has sought forgiveness, even if he returns to it seventy times in a day."