Sayyiduna Zayd bin Khalid al-Juhani, Abu Hurayrah and Shibl (may Allah be pleased with them) relate: We were with the Prophet (peace and blessings be upon him) when a man asked about a slave woman who commits adultery while unmarried, and he (peace and blessings be upon him) said: "Flog her." He asked: And if she commits adultery again? He said: "Flog her." Then, concerning the third or the fourth time, he said: "Sell her, even if for a rope of hair."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5184