From Sahih Muslim (Hadith 4445):
Abu Huraira reported that he heard Allah's Messenger (ﷺ) as saying:When the slave-woman of any of you commits adultery and this (offence of hers) becomes clear, she should be flogged (as the prescribed) punishment, but hurl no reproach at her. If she commits adultery again, she should (again be punished) by flogging, but hurl no reproach upon her. If she commits fornication for the third time and it becomes clear, then he should sell her, even if only for a rope of hair
Reference: Muslim 4445