Musannaf Ibn Abi Shayba 28:639

Waki’ narrated to us, he said: Sufyan narrated to us, from Jabir, who said: I asked ‘Amir about the Muslims who go on a military expedition with the people of the Dhimma (protected non-Muslims), and they distribute [spoils] to them and waive their Jizya (poll tax) from them. He said: That is a good voluntary gift (nafl) for them.