In this campaign the Muslims captured a large number of war prisoners. As Abu Darda saw these prisoners, Abu Darda wept bitterly. Some one asked Abu Darda as to why he was weeping when it was an occasion for the Muslims to rejoice. Abu Darda said, "I am weeping at the fate of people who disobey God. When they disobey God they come to grief like these people. I fear for the Muslims, lest by disobeying God they may also invite such a grim fate for themselves. "
"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