Musnad Ahmad 215:5

Abu al-Nadr narrated to us, al-Mubarak narrated to us, from al-Hasan, from Salamah ibn al-Muhaibaq, who said: The Messenger of Allah (may Allah bless him and grant him peace) was asked about a man who has intercourse with his wife’s slave girl. He said: "If he forced her, then she is free, and she is entitled to a like of her from him. And if she consented to him, then she is his slave girl, and she is entitled to a like of her from him."