Musnad Abu Ya'la al-Mawsili 160:213

Yahya ibn Ayyub narrated to us, Sufyan narrated to us, from Ayyub, from Sa'id ibn Abi Sa'id, from Abu Hurayrah, from the Prophet (peace be upon him) who said: "If the slave-girl of one of you commits adultery, and her adultery is proven, then let him flog her and not reproach her. Then if she commits adultery, let him flog her the prescribed punishment and not reproach her." Sufyan said: "Meaning, do not shame her." "Then if she commits adultery, let him flog her the prescribed punishment and not reproach her. Then if she commits adultery, let him sell her, even if for a rope."