Sahih Bukhari, Chapter: Sales and Trade

I heard the Prophet saying, "If a slave-girl of yours commits illegal sexual intercourse and her illegal sexual intercourse is proved, she should be lashed, and after that nobody should blame her, and if she commits illegal sexual intercourse the second time, she should be lashed and nobody should blame her after that, and if she does the offense for the third time and her illegal sexual intercourse is proved, she should be sold even for a hair rope."