The Messenger of Allah (sal Allahu alaihi wa sallam) said: “Save yourself from anger, because it is a flaming fire which burns the heart of the son of Adam.” [Rooh ul Ma’aani]
After attaining Emaan remaining firm on it is called steadfastness.
There are many who act like the friends of Allah for one moment but the next moment they turn into devils. For a few days they are like angels, but when the self (Nafs) overpowers them, they become like devils. When anger overpowers them they do not think who they are and who their Allah is. Then they even forget that a little while ago they were reciting the Quran, last night they performed Tahajjud, and even prayed Ishraq this morning. In anger, they change into Shaytaan, say what they want to, and even beat someone up!
Why is such a person called a Shaytaan? Because Shaytaan is created from fire and anger is also created from fire.
That is why the Prophet (sal Allahu alaihi wa sallam) recommended the person who feels himself to be getting angry, to drink water, since water quenches fire. Get away from the person making you angry, do Wudhu, splashing yourself well with water, and pray two Rakat Nafl asking Allah (subhana wa ta’ala) for help to subdue your anger.
Allamah Abul Qasim Qushairi said: “A friend of Allah does not take revenge, and one who takes revenge can never be a friend of Allah.”
[Peace of Mind by Hakeem Muhammad Akhtar]