Musannaf Ibn Abi Shayba 19:203

Abu Bakr narrated to us, he said: Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from 'Ubayd Allah, from Abu Hurayrah, Zayd ibn Khalid, and Shibl; they said: We were with the Prophet, may God bless him and grant him peace, when a man asked him about a slave woman who commits adultery before she is married. He said: "Flog her. Then if she commits adultery, flog her." He said: "In the third or the fourth [time], sell her, even if for a rope of hair."