Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4691
It has been narrated on the authority of Abdullah b. Amr (through a different chain of transmitters) that the Messenger of Allah (may peace be upon him) said: A troop of soldiers, large or small, who fight (in the way of Allah), get their share of the booty and return safe and sound, receive in advance two-thirds of their reward (only one-third remaining to their credit to be received in the Hereafter) ; and a troop of soldiers, large or small, who return empty-handed and are afflicted or wounded, will receive their full reward (in the Hereafter).