The Mudabbar
Malik :: Book 41 : Hadith 41.3.14
Malik related to me from Ibn Shihab from Ubaydullah ibn Abdullah ibn Utba ibn Masud from Abu Hurayra and Zayd ibn Khalid al-Juhani that the Messenger of Allah, may Allah bless him and grant him peace, was asked about a slave-girl who committed fornication and was not muhsana. He said, "If she commits fornication, then flog her. If she commits fornication again, then flog her, and if she commits fornication again, then sell her, if only for a rope."
Ibn Shihab added, "I don't know whether it was three or four times."