Musnad Ahmad 738:2

Yazid ibn Abd Rabbih narrated to us, Baqiyyah ibn al-Walid narrated to us, al-Zubaydi narrated to me, from al-Zuhri, from Ubayd Allah ibn Abd Allah, that Shibl ibn Khulayd al-Muzani informed him, that Abd Allah ibn Malik al-Awsi informed him, that the Prophet, peace and blessings of Allah be upon him, said to the slave girl: "If she commits adultery, then flog her. Then if she commits adultery, then flog her. Then if she commits adultery, then flog her. Then if she commits adultery, then sell her, even if for a rope." And the *dhafir* (rope) is the rope.