From Sahih Muslim (Hadith 4447):
Abu Huraira reported that Allah's Messenger (ﷺ) was asked about the slave-woman who committed adultery and was not protected (married). He said:If she commits adultery, then flog her and if she commits adultery again, then flog her and then sell her even for a rope. Ibn Shihab said: I do not know whether he said this (his statement pertaining to the sale of slave-woman) at the third or the fourth time. Ibn Shihab said that the word dafir (used in the text) means rope
Reference: Muslim 4447