Musannaf Ibn Abi Shayba 35:41

Ibn ‘Uyaynah narrated to us, from Ayyub ibn Musa, from Sa‘id, from Abu Hurayrah, who said: The Prophet (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 sell her, even if for a strand of hair."