Sunan an-Nasa'i 3363

It was narrated that Salamah bin Al-Muhabbaq said:"The Prophet passed judgment concerning a man who had intercourse with his wife's slave woman: 'If he forced her, then she is free, and he has to give her mistress a similar slave as a replacement; if she obeyed him in that, then she belongs to him, and he has to give her mistress a similar slave as a replacement