Musnad Ahmad 27:756

Waki' 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: "Allah the Exalted does not accept a charity from ghulul (ill-gotten gains/spoils of war taken before distribution), nor a prayer without purification."