Musannaf Ibn Abi Shayba 13:3179

Abu Bakr narrated to us, he said: Zayd ibn al-Hubab narrated to us, from Musa ibn Ali ibn Rabah, who said: I asked al-Zuhri about a *mukatab* (an enslaved person with a contract of manumission) who married a free woman and she bore him a child, and he bought a slave woman and she bore him a child, then he died while something of his *kitabah* (contractual payment) remained. Which of the two [children] should work to pay off what remains of it? He said: His child who is from his slave woman.