Musnad Abu Ya'la al-Mawsili 1:137

'Abd al-Ghaffar ibn 'Abd Allah ibn al-Zubayr narrated to us, 'Afif ibn Salim narrated to us, from 'Uthman ibn Waqid, who said: Abu Nasirah narrated to me, from a freed slave of Abu Bakr, from Abu Bakr, who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Whoever seeks forgiveness, then does not persist, even if he returns to it seventy times in a day."