Musnad Ahmad 30:2343

Yahya ibn Sa'id ibn Aban ibn Sa'id ibn al-'As narrated to us, he said: 'Ubayd Allah narrated to us, from Sa'id ibn Abi Sa'id, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If the servant of one of you commits adultery, let him flog her and not reproach her. If she repeats it a second time, let him flog her and not reproach her. If she repeats it a third time, let him flog her and not reproach her. If she repeats it a fourth time, let him flog her and sell her for a rope of hair," or "for a braid of hair."