Musnad Ahmad 27:1790

Hajjaj narrated to us, Layth narrated to us, from Uqayl, from Ibn Shihab, from Salim ibn Abdullah, from Abdullah ibn Umar: That the Messenger of Allah (peace and blessings of Allah be upon him) used to grant extra spoils (nafal) to some of those he sent out in military detachments (saraya) for themselves specifically, apart from the share of the general army. And the fifth (al-khums) in that is obligatory for Allah the Exalted.