Musnad Ahmad 795:5

Isma'il narrated to us, from Yunus, from al-Hasan, from Salamah ibn al-Muhaibaq, that a man went out on a military expedition, and with him was a slave girl belonging to his wife, and he had intercourse with her. This was mentioned to the Prophet, peace and blessings of Allah be upon him, and he said: "If he coerced her, then she is free, and she has [the right to] one like her from him. And if she consented to him, then she is his slave girl, and she has [the right to] one like her from him." And Isma'il said once: That a man was on a military expedition.