Musnad Ahmad 30:2350

Abu Mu'awiyah Muhammad ibn Khazim narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah has guaranteed for the one who goes out in His cause that He will admit him into Paradise, or return him to his dwelling, having attained what he attained of reward or spoils of war."