Amir bin Matar al-Shaybani narrated that Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said concerning a man who has intercourse with his wife's slave woman: "If he forced her, she becomes free, and he must pay compensation of a slave woman like her; and if the slave woman went along with him willingly, the man shall keep her himself and pay compensation of a slave woman like her."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9652