He would not pay the enemy in its own coin
At the battlefield of Siffin, the forces of Muawiyah had the control over the source of water supply, and they would not allow thereto the troops of Ali. When the troops of Ali overpowered the enemy and got control over the source of water supply, Ali was advised that he should deny access of water to the enemy. He repudiated the advice saying that he would not pay the enemy in its own coins, but in the coins of Islam.
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"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