Musannaf Ibn Abi Shayba 13:1556

Abu Bakr narrated to us, he said: Muhammad ibn Yazid narrated to us, from Abu al-Ala, from Qatadah, that Umar ibn al-Khattab and Zayd ibn Thabit said: "If a mukatab (an enslaved person under a contract of manumission) dies and he has wealth, it belongs to his masters, and his children have nothing."