Maliks Muwatta Book 31, Hadith Number 6.
Yahya related to me from Malik from Nafi that Abdullah ibn Umar would say, “A man should not have intercourse with a slave girl except one whom, if he wished, he could sell, if he wished, he could give away, if he wished, he could keep, if he wished, he could do with her what he wanted.”
Malik said that a man who bought a slave-girl on condition that he did not sell her, give her away, or do something of that nature, was not to have intercourse with her. That was because he was not permitted to sell her or to give her away, so if he did not own that from her, he did not have complete ownership of her because an exception had been made concerning her by the hand of someone else. If that sort of condition entered into it, it was a messy situation, and the sale was not recommended.