Musannaf Ibn Abi Shayba 35:42

Shababa narrated to us, from Layth ibn Sa'd, from Yazid ibn Abi Habib, from 'Umarah ibn Abi Farwah, from 'Urwah, from 'Aishah, that the Prophet (may Allah bless him and grant him peace) said: "If the slave woman commits adultery, then flog her. If she repeats it, then flog her. If she repeats it, then flog her. If she commits adultery [again], then flog her, then sell her, even if for a *dhafir*." And the *dhafir* is a rope.