Musannaf Ibn Abi Shayba 1:26

Husayn ibn Ali narrated to us from Za'idah, and Waki' narrated to us from Isra'il, both of them from Simak ibn Harb, from Mus'ab ibn Sa'd, from Ibn Umar, who said: The Messenger of Allah (peace be upon him) said: "Prayer is not accepted without purification, nor is charity [accepted] from spoils of war [taken] dishonestly."