Muwatta Malik (Yahya)

Imam Malik, may Allah have mercy on him, said regarding a man who marries a slave woman, then that slave woman gives birth to his child, and then he buys her: (he said that) indeed, that slave woman will not become an umm walad (mother of a child) for him because of the child she bore when she was in the ownership of another person. (She cannot become umm walad) until she gives birth to his child while she is in his ownership after he has purchased her.

Imam Malik, may Allah have mercy on him, said: And if (the husband) buys her (the slave woman) in such a state that she has already become pregnant from his semen, then if she gives birth to a child after coming into his ownership, according to our opinion, she will be considered Umm Walad because of that pregnancy. And Allah knows best.

And he narrated to me from Malik that he asked Ibn Shihab about a man who had a slave woman as his wife, then he bought her, and he had divorced her once. He said: "She is lawful for him by virtue of his right hand's ownership as long as he has not made her divorce final. But if he has made her divorce final, then she is not lawful for him by virtue of his right hand's ownership until she marries another husband besides him."