Amr bin Shu'ayb narrates from his father, from his grandfather (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) used to give extra awards from the spoils before the khumus (one-fifth) was made obligatory. When the verse "whatever you take as spoils of anything, one-fifth of it is for Allah" [Surah al-Anfal: 41] was revealed, he stopped giving in the way he used to, and that (extra award) came to be taken from the fifth of the khumus, from the share of Allah and the share of the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12811