Musnad Ahmad 1174:1

Ishaq ibn Ibrahim al-Razi narrated to us, he said: Salamah ibn al-Fadl narrated to us, he said: Muhammad ibn Ishaq narrated to me, from al-Khattab ibn Salih, from his mother, she said: Salamah bint Ma'qal narrated to me, she said: I belonged to al-Hubab ibn 'Amr, and I had a son by him. His wife said to me: "You will now be sold to pay his debt." So I came to the Messenger of Allah, peace and blessings of Allah be upon him, and mentioned that to him. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Who is the heir of the estate of al-Hubab ibn 'Amr?" They said: "His brother, Abu al-Yusr Ka'b ibn 'Amr." So the Messenger of Allah, peace and blessings of Allah be upon him, called him and said: "Do not sell her, and set her free. When you hear that slaves have come to me, come to me and I will compensate you for her." They did so. Then they differed among themselves after the death of the Messenger of Allah, peace and blessings of Allah be upon him. Some people said: "The mother of the child (umm al-walad) is a slave; were it not for that, the Messenger of Allah, peace and blessings of Allah be upon him, would not have compensated them for her." Others said: "She is free; the Messenger of Allah, peace and blessings of Allah be upon him, set her free." And that is where the disagreement lay.