From Sahih Bukhari (Hadith 2556):
Narrated Abu Huraira and Zaid bin Khalid:The Prophet (ﷺ) said, "If a slave-girl (Ama) commits illegal sexual intercourse, scourge her; if she does it again, scourge her again; if she repeats it, scourge her again." The narrator added that on the third or the fourth offense, the Prophet (ﷺ) said, "Sell her even for a hair rope
Reference: Bukhari 2556