Musannaf Ibn Abi Shayba 13:2168

Abu Bakr narrated to us, he said: Ibn Abi Za'ida narrated to us, from Musa ibn Ubaydah, from Abdullah ibn Dinar, from Ibn Umar, he said: The Prophet, may Allah bless him and grant him peace, forbade that a *kali'* be sold for a *kali'*, meaning a debt for a debt.