Narrated Abdullah ibn Umar: The Messenger of Allah ﷺ used to give to some of the detachments he sent out (something extra) for themselves in particular apart from the division made to the whole army. The fifth is necessary in all that.
Hadith Referenceسنن ابي داود / كتاب الجهاد / 2746
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح بخاري (3135) صحيح مسلم (1750)
1؎: That is, first the khums (one-fifth) would be taken out from the entire wealth, then if there was anything to be given as reward, it would be given, and then the remainder would be distributed equally among the general army.
Narrated Ibn `Umar: Allah's Apostle used to give extra share to some of the members of the Sariya he used to send, in addition to the shares they shared with the army in general.
Shaykh Abdul Sattar al-Hammad
Hadith Commentary: ➊ In the terminology of Islamic law, "nafl" refers to the gift that the Imam gives to a person who has shown some achievement in battle.
➋ From the first hadith, it is understood that it is permissible to give a nafl from the spoils of war in addition to the allotted share, because it is explicitly mentioned that the warriors were given, as a reward, an extra camel each besides their share of the spoils. The Messenger of Allah (sallallahu alayhi wa sallam) did not object to this distribution.
➌ According to the second hadith, some warriors may be given a greater share than others due to their good performance. However, this additional share will be given from the khums (one-fifth portion). (: ‘Umdat al-Qari: 404/10)
➍ The purpose of Imam Bukhari (rahimahullah) is to show that the Imam is authorized to distribute the wealth of khums at his own discretion. He may give more to someone due to their distinguished services. And Allah knows best.
Source: Hidayat al-Qari: Commentary on Sahih Bukhari, Urdu, Page: 3135