Musnad Ahmad 30:1765

Muhammad ibn Ubayd narrated to us, Ubayd Allah narrated to us, from Sa’id ibn Abi Sa’id, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If the slave-girl of one of you commits adultery, let him flog her and not reproach her. If she repeats it, let him flog her and not reproach her. If she repeats it, let him flog her and not reproach her. If she repeats it a fourth time, let him sell her, even if for a rope of hair or a braid of hair."