What is the source of the discretionary reward (Radkh) given for participation in battle?

Primary text

There are two opinions regarding the source of the Radkh. The first view holds that it comes from the main body of the spoils (Ghanimah), as it is a reward for assisting in securing the spoils, similar to the wages of transporters and guards. The second view asserts that it comes from the four-fifths share allocated to the combatants, as it is compensation for being present at the battle, resembling the fixed shares of the warriors.