Vishnu Purana 1 - only fools remain angry

When the Sage Parashar's father, Sakti Maharsi, was murdered and eaten by a Rakshasa (demon) named Rudhir. Rudhir had been a human King, but had been transformed into a demon by Vishwamitra. Yet Sage Parashar grew angry at the demons. In his anger, he began to systematically destroy every demon, seeking to rid every world of demons. He lit a great sacrificial fire, and presented as an offering every demon he captured or killed. Soon, the demons feared extinction. The demons asked the help of Sage Parashar's grandfather, Vasishtha, and he agreed he would talk to his grandson.

Grandfather told grandson that too much anger was not good. Not all demons could be blamed for his father's death. He instructed his grandson in the nature of Demons, their place and necessity to the world, and the good things they sometimes did. He explained that in fact, no demon was responsible for his father's death: his father had earned an untimely death by his war with Vishwamitra. His war with Vishwamitra was perpetuated through every world, across all of time, because each combatant sustained their anger against the other. Grandfather told grandson, "a human being naturally gets angry, but only fools remain angry. Sannyasis should not hold on to anything, but should renounce everything. Sannyasis should not hold onto anger. Shun your anger, Sannyasi. Stop your war against innocent demons. Forgive them, and let go of your anger."

Thus convinced by his grandfather's instruction, Sage Parashar extinguished the flame of revenge, stopped his war, and propagated his grandfather's Dharma, teaching anyone the same lesson he himself learned. "A human being always bears the consequences their actions, good and bad. Anger destroys all the results of a Sannyasi's renunciation. This is why Sannyasis shun anger." When he began to teach this, Brahma's son Pulastya arrived on the spot and, presenting him with a copy of the Puranas, and being able to trust him with the knowledge therein, instructed him in it.