Sahih Muslim | Book 19 | Hadith Number 4337

Narrated by Abdullah b. 'Umar
It has been narrated on the authority of Abdullah b. 'Umar that the Messenger of Allah (may peace be upon him) used to give (from the spoils of war) to small troops seat on expeditions something more than the due share of each fighter in a large force. And Khums (one-fifth of the total spoils) was to be reserved (for Allah and His Apostle) in all cases.