Messenger of Allah ﷺ was asked about the slave-girl, if she was a virgin and committed illegal sexual intercourse. The Prophet ﷺ 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."
أَنَّ رَسُولَ اللَّهِ ﷺ سُئِلَ عَنِ الأَمَةِ إِذَا زَنَتْ وَلَمْ تُحْصِنْ قَالَ إِنْ زَنَتْ فَاجْلِدُوهَا ثُمَّ إِنْ زَنَتْ فَاجْلِدُوهَا ثُمَّ إِنْ زَنَتْ فَبِيعُوهَا وَلَوْ بِضَفِيرٍ قَالَ ابْنُ شِهَابٍ لاَ أَدْرِي بَعْدَ الثَّالِثَةِ أَوِ الرَّابِعَةِ