Musnad Ahmad 25:1051

Hajjaj narrated to us, Shareek narrated to us, from Husayn ibn Abdullah, from Ikrimah, from Ibn Abbas, from the Prophet (peace and blessings of Allah be upon him), who said: "Any slave woman who has borne a child to her master, she is emancipated after his death," or he said, "from after him," and perhaps he said both of them.