Musannaf Ibn Abi Shayba 13:1634

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Salamah ibn Kuhayl, from Zayd ibn Wahb, who said: Umar ibn al-Khattab sold slave mothers (umm al-walad) among us, then he returned them to us, until he returned them pregnant from Tustar.