Musannaf Ibn Abi Shaybah

Ibrahim and Hazrat Amir, may Allah have mercy on them, were asked: If a man marries a slave woman, then she bears him a child, and then he also buys her, what is the ruling regarding this? Both of them replied: He can sell her as long as she has not given birth to a child in his ownership.

Hazrat Tawus says that he can buy (sell) it.

Hazrat Hasan (may Allah be pleased with him) says that she is his umm walad.

Hazrat Hammad says: He should not sell her; she is his Umm-e-Walad.