Musannaf Ibn Abi Shayba 9:349

'Abdullah ibn al-Mubarak narrated to us, from Musa ibn Ayyub, from his paternal uncle, from 'Ali, he said: I asked him about a man who has two slave girls who are sisters; he had intercourse with one of them, then he wanted to have intercourse with the other. He said: "No, not until he removes her from his ownership." I said: "What if he marries her to his slave?" He said: "No, not until he removes her from his ownership."