Mukhtasar Bukhari

Abu Huraira (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When a slave-girl commits adultery and it is proven against her, she should be punished and not just reprimanded. If she commits adultery again, she should be punished again and not just reprimanded. Then if she commits adultery a third time, she should be sold, even if it is for a rope of hair."