Musnad Abu Ya'la al-Mawsili 160:146

Abu Khaythama narrated to us, Sufyan ibn 'Uyayna narrated to us, from Ayyub ibn Musa, from Sa'id ibn Abi Sa'id, from Abu Hurayra, from the Prophet (may Allah bless him and grant him peace), who said: "If the slave-girl of one of you commits adultery, and her adultery is proven, then let him inflict the prescribed punishment (hadd) upon her, and let him not reproach her. Then if she commits adultery, let him inflict the prescribed punishment upon her, and let him not reproach her. Then if she commits adultery, let him inflict the prescribed punishment upon her, and let him not reproach her. Then if she commits adultery, let him sell her, even if for a rope of hair."