Musnad Ahmad 738:1

Ya'qub narrated to us, Ibn Akhi Ibn Shihab narrated to us, from his uncle, who said: 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah ibn Mas'ud informed me that Shibl ibn Khulayd al-Muzani informed him that 'Abd Allah ibn Malik al-Awsi informed him that the Messenger of Allah (peace and blessings of Allah be upon him) said regarding a 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 *dafir*." And the *dafir* is a rope, in the third or the fourth [instance].