Musannaf Ibn Abi Shayba 13:1637

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Abdullah ibn Dinar, who said: It was said to Ibn Umar, "Ibn al-Zubayr is selling the *umm al-walad* (slave women who have borne children to their masters)." Ibn Umar said, "But Umar decreed that they are not to be sold, nor are they to be gifted. Their master may enjoy them during his lifetime, and when he dies, she is free."