It is narrated from Sayyiduna Salamah bin al-Muhabbiq (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said concerning a man who had intercourse with his wife's slave-girl: "If he forced her, she becomes free, and he owes the like of her (as compensation); and if she yielded to him willingly, she belongs to that husband, and he owes his wife the like of her."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6306