Musnad Ahmad 795:1

Affan narrated to us, Hammad bin Zaid narrated to us, Amr bin Dinar narrated to us, he said: I heard al-Hasan, from Salamah bin al-Muhaibaq, that a man had intercourse with his wife’s slave girl. That was raised to the Prophet, peace and blessings of Allah be upon him, and he said: "If she obeyed him, then she is for him, and upon him is the like of her for her. And if he forced her, then she is free, and upon him is the like of her for her."