From Sahih Bukhari (Hadith 2232):
Narrated Zaid bin Khalid and Abu Huraira:that Allah's Messenger (ﷺ) was asked about an unmarried slave-girl who committed illegal sexual intercourse. They heard him saying, "Flog her, and if she commits illegal sexual intercourse after that, flog her again, and on the third (or the fourth) offense, sell her
Reference: Bukhari 2232