Sayyiduna Abu Hurayrah and Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with them both) relate that they heard the Messenger of Allah (peace and blessings be upon him), when he was asked about a slave woman who commits adultery while unmarried, say: "If she commits adultery, flog her; then if she commits adultery, flog her; then, the third or the fourth time, sell her, even if for a rope of hair."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5187