Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "There is no army that fights in the path of Allah and gains spoils, except that they receive two thirds of their reward in advance and one third remains; and if they gain no spoils, they receive their reward in full."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13674