After the conquest of Khyber, the Muslims marched to Fidak, another Jewish settlement, not far from Khyber. Having seen the fate of the people of Khyber, the people of Fidak did not choose to fight. They agreed to execute a treaty of peace with the Muslims on the same terms as had been agreed upon in the case of the Jews of Khyber. As Fidak had fallen without fight, the annual tribute therefrom was earmarked as the private purse of the Holy Prophet. After meeting the household expenses of the Holy Prophet the proceeds were to be utilized for the welfare of the widows, the orphans, the needy, the distressed and the wayfarers.
"When a person dies, his works end, except for three: ongoing charity, knowledge that is benefited from, and a righteous child who prays for him."
Prophet Mohammed (PBUH)
"The best of what a man leaves behind are three: a righteous child who supplicates for him, ongoing charity the reward of which reaches him, and knowledge that is acted upon after him."
Sunan Ibn Mājah
"Every day two angels come down from Heaven and one of them says, 'O Allah! Compensate every person who spends in Your Cause,' and the other (angel) says, 'O Allah! Destroy every miser.'"
Sahih Bukhari