Sahih Bukhari, Chapter: Sales and Trade

Allah's Apostle was asked about the slave-girl, if she was a virgin and committed illegal sexual intercourse. The Prophet said, "If she committed illegal sexual intercourse, lash her, and if she did it a second time, then lash her again, and if she repeated the third time, then sell her even for a hair rope." Ibn Shihab said, "I don't know whether to sell her after the third or fourth offense."