Sahih Muslim , Book 19, Hadith 4337 (USC-MSA) ⓘ

It has been narrated on the authority of Abdullah b. 'Umar that the Messenger of Allah 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.

وَحَدَّثَنَا عَبْدُ الْمَلِكِ بْنُ شُعَيْبِ بْنِ اللَّيْثِ، حَدَّثَنِي أَبِي، عَنْ جَدِّي، قَالَ حَدَّثَنِي عُقَيْلُ، بْنُ خَالِدٍ عَنِ ابْنِ شِهَابٍ، عَنْ سَالِمٍ، عَنْ عَبْدِ اللَّهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَدْ كَانَ يُنَفِّلُ بَعْضَ مَنْ يَبْعَثُ مِنَ السَّرَايَا لأَنْفُسِهِمْ خَاصَّةً سِوَى قَسْمِ عَامَّةِ الْجَيْشِ وَالْخُمْسُ فِي ذَلِكَ وَاجِبٌ كُلِّهِ ‏.‏