From Sahih Muslim (Hadith 4565):
It has been narrated on the authority of Abdullah b. 'Umar that the Messenger of Allah (ﷺ) 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
Reference: Muslim 4565