Musnad Abu Ya'la al-Mawsili 153:200

'Amr ibn Muhammad al-Naqid narrated to us, Waki' ibn al-Jarrah narrated to us, Isra'il narrated to us, from Simak ibn Harb, from Mus'ab ibn Sa'd, from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Prayer is not accepted without purification, nor is charity [accepted] from spoils of war [taken] dishonestly."