He would not accept the allegiance from whom he had released
Marwan a cousin of Muawiyah was a bitter enemy of Ali. He had played a great part in inciting revolt against the caliphate of Ali. He was taken captive in the Battle of the Camel at Basra. When brought before Ali he asked for mercy. Ali released him unconditionally. Thereupon Marwan offered to pay allegiance to him, but Ali said, "I did not want his allegiance. I released him for the sake of God, and not for securing allegiance to me."
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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
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Sahih Bukhari