Sayyiduna Salamah bin al-Muhabbiq (may Allah be pleased with him) narrates: The Messenger of Allah (peace and blessings be upon him) judged concerning a man who had intercourse with his wife's slave-girl: "If he forced her, she becomes free, and he owes her mistress the like of her; and if she yielded to him willingly, she remains his slave-girl, and he owes her mistress the like of her."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6307