It is narrated from Ibrahim (may Allah have mercy on him) that a man married a slave woman and she had other slave women with her. Whenever he touched one of them, she said: "I am not your wife." This happened three times, then he had relations with her. When he saw, she was not his wife. Ibrahim (may Allah have mercy on him) said: "The dowry is obligatory for that woman, and the legal punishment is waived for the man because he did not know."
Hadith Referenceسنن سعید بن منصور / كتاب النكاح / 2189