Sahih Bukhari Volume 8, Book 82, Hadith Number 823.

Narrated By Abu Huraira : The Prophet said, “If a lady slave commits illegal sexual intercourse and she is proved guilty of illegal sexual intercourse, then she should be flogged (fifty stripes) but she should not be admonished; and if she commits illegal sexual intercourse again, then she should be flogged again but should not be admonished; and if she commits illegal sexual intercourse for the third time, then she should be sold even for a hair rope.”