It is narrated from Sayyiduna Salamah bin al-Muhabbiq (may Allah be pleased with him) that a man from Quraysh went on a journey, and his wife sent a slave-girl of hers with him. He had intercourse with her. Then when he came to the Prophet (peace and blessings be upon him), his wife told him (peace and blessings be upon him) about it. He (peace and blessings be upon him) said: "If she yielded to him willingly, she is his, and he owes the like of her; and if he forced her, she becomes free, and he owes the like of her."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6315