Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When a slave-woman commits zina and her zina becomes clear, flog her with the prescribed punishment and do not reproach her. If she commits zina a second time, flog her again; if she commits zina a third time, sell her, even if you sell her for a rope of hair."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17086