Musannaf Ibn Abi Shayba 18:1278

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from ‘Isa ibn Abi ‘Izzah, from al-Sha’bi, regarding His saying: "And if he is from a people between whom and you there is a treaty," he said: "From the people of the covenant (ahl al-‘ahd), and he is not a believer."