Musannaf Ibn Abi Shayba 13:2863

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from Yazid ibn Ibrahim, from Ibn Sirin, who said: I was informed that a son of Ibn Umar said to him: "It is disliked to separate between a slave woman and her sister, yet you have separated between me and my mother."