Musnad Ahmad 30:3275

Hajjaj narrated to us, Laith narrated to us, Sa'id ibn Abi Sa'id narrated to me, from his father, from Abu Hurairah, that he heard him say: I heard the Messenger of Allah (ﷺ) say: "If the slave-girl of one of you commits adultery, and her adultery is proven, then let him lash her with the prescribed punishment, and let him not reproach her. Then if she commits adultery, let him lash her with the prescribed punishment and let him not reproach her. Then if she commits adultery and her adultery is proven, let him sell her, even if for a rope of hair."