Musnad Ahmad 30:277

Sufyan narrated to us, from Ayyub ibn Musa, from Sa’id, from Abu Hurayrah, from the Prophet, peace be upon him: "If the slave-girl of one of you commits adultery, and her adultery is proven, let him inflict the prescribed punishment (hadd) upon her, and let him not reproach her." Sufyan said: "Do not reproach her" means: do not shame her for it. "In the third or fourth [time]: let him sell her, even if for a rope of hair."