Maliks Muwatta Book 28, Hadith Number 32.

Yahya related to me from Malik that he had asked Ibn Shihab about a man who had a slave-girl as a wife, and then he bought her, and divorced her once. He said, “She is halal for him by the possession of the right hand as long as he does not make his divorce irrevocable. If he irrevocably divorces her, she is not halal for him by the possession of the right hand until she has married another husband.”

Malik said that if a man married a female slave and then she had a child by him, and then he bought her, she was not an umm walad for him because of the child born to him while she belonged to another, until she had had a child by him while she was in his possession after he had purchased her.

Malik said, “If he buys her and she is pregnant by him and she then gives birth while she belongs to him, she is his umm walad by that pregnancy, according to what we think, and Allah knows best.”