Musnad Ahmad 456:29

'Abd al-Rahman ibn Mahdi narrated to us, he said: Malik narrated to us, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah, from Zayd ibn Khalid al-Juhani and Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) was asked about a slave woman who commits adultery and has not been married (i.e., is not *muhsan*). He said: "Flog her. Then if she commits adultery, flog her." He said in the third or the fourth time: "Then if she commits adultery, sell her, even if for a rope." And the *dhafir* (rope) is the cord.