Musnad Ahmad 795:4

Abdullah ibn Bakr narrated to us, Sa’id—meaning Ibn Abi Arubah—narrated to us, from Qatadah, from al-Hasan, from Salamah ibn al-Muhaibaq, that a man had intercourse with his wife’s slave girl while he was on a military expedition. This was reported to the Prophet, peace and blessings of Allah be upon him, and he said: "If he coerced her, then she is free at his expense, and he must purchase a replacement for her mistress. And if she consented to him, then a like of her from his wealth is due to her mistress."