Musnad Abu Ya'la al-Mawsili 154:47

Masruq ibn al-Marzuban narrated to us, Ibn Abi Za'idah narrated to us, from Isra'il, from Simak, from Mus'ab ibn Sa'd, from Ibn 'Umar, from the Prophet (may Allah bless him and grant him peace), who said: "Prayer is not accepted without purification, nor is charity [accepted] from spoils of war taken dishonestly (ghulul)."