Musnad Abu Ya'la al-Mawsili 153:165

Abu Khaythama narrated to us, Ishaq ibn ‘Isa narrated to us, Layth narrated to us, ‘Aqil narrated to me, from al-Zuhri, from Salim, from his father: That the Messenger of Allah (peace and blessings of Allah be upon him) used to grant a *nafal* (extra share of spoils) to some of those he sent out in military detachments, specifically for themselves, as an extra share apart from the distribution to the general army. And the *khums* (one-fifth) is obligatory in all of that.