Musannaf Ibn Abi Shayba 13:1324

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Al-Hasan ibn Salih narrated to us, from Al-Hasan ibn 'Amr, from Fudayl, from Ibrahim, he said: A slave is not to be sold for a debt, even if he owes one hundred thousand.