According to a tradition, a Companion came to the Prophet and asked him a question. He said, "O Messenger of God, teach me some words that I can live by, and it should not be lengthy, lest I forget." (Muwatta Imam Malik 3362) That is: " يَا رَسُولَ اللهِ عَلِّمْنِي كَلِمَاتٍ أَعِيشُ بِهِنَّ. وَلاَ تُكْثِرْ عَلَيَّ فَأَنْسَى." So he replied:(لا تغضب) "Do not get angry." If the words of this Hadith are changed, it would be as if the questioner had asked: “O Prophet, give me a master advice by which I may be able to manage all the affairs of my life.” 

It means that he asked for a key—the key of life, the key of a successful life. So the Prophet said, "Leave anger; this is the key to success." What's so wrong with anger? I thought about it that why is anger so bad? Then, I found the answer in a hadith. This hadith is in Al-Bayhaqi.

The words of the hadith are: انَّ الْغَضَبَ لِيُفْسِدُ الْإِيمَان—Anger ruins a person's faith. Anger corrupts a person's faith. (Shu'abul Iman by Al-Baihaqi 7941) Here, the word "faith" does not simply mean a formal belief; it is in the context of divine wisdom. The meaning of this is that anger corrupts the Divine wisdom. God has placed within your nature the divine wisdom which is supported by faith. It is jeopardised when one becomes angry. Upon reflection, I realized the harm of anger. It brings rational thinking to an end. Consider all the events in history associated with anger, and you'll find that anger stops right thinking. We learn from a Quranic verse that even if a Prophet becomes angry, within him too, God forbid, right thinking can cease. This is the example of Hazrat Yunus.

Hazrat Yunus was a prophet. He was sent to preach in Nineveh, a city in Iraq. In those days, it was the center of Iraq. There, he invited them, but the people did not heed his words. So, in haste and anger, he left the place, abandoning the town. The Quran states in chapter Al-Anbiya: Remember the man in the whale [Jonah] when he went away in anger, thinking We had no power over him..وَذَا النُّونِ إِذْ ذَهَبَ مُغَاضِبًا فَظَنَّ أَنْ لَنْ نَقْدِرَ عَلَيْهِ (21:87) This shows that he left the town in anger. So what happened? It means that God Almighty did not approve of his action; he was swallowed by a fish. Then he sought forgiveness, and once again he returned to Nineveh, where he again began calling people to God.