Abu Dawud Book 33, Hadith Number 4445.

Narated By Salamah ibn al-Muhabbaq : The Apostle of Allah (PBUH) made a decision about a man who had intercourse with his wife’s slave-girl as follows. If he forced her, she is free, and he shall give her mistress a slave-girl similar to her; if she asked him to have intercourse voluntarily, she will belong to him, and he shall give her mistress a slave-girl similar to her.