سنن النسائي

It was narrated from Salamah bin Al-Muhabbaq that a man had intercourse with a slave woman belonging to his wife, and was brought to the Messenger of Allah. He said: "If he forced her, then she is free at his expense and he has to give her mistress a similar slave as a replacement. If she obeyed him in that, then she belongs to her mistress, and he has to give her mistress a similar slave as well."