The Messenger of Allah (S) has said, “O' ‘Ali! I advise you (in regards to something) with a piece of advice, so then safe-guard this as you shall never be devoid of goodness as long as you have safe-guarded my recommendation. O' ‘Ali! Allah will grant the one who swallows his anger - while he is able to act out his rage - with protection and faith on the Day of Judgment whose pleasure the person will taste.”[1]
In this piece of advice from the Messenger of Allah (S), there is only one thing which he advised us. In this tradition, he stated that if a person has the ability to do something (take revenge) and refrains from it, Allah (SwT) will grant that person a blessing in the transient world and the next life. The blessing of the next life is the protection from the hell-fire, while the blessing in this world is that of true faith which the person shall be able to taste.
The word “كظم” which has been mentioned in this tradition has numerous meanings, however its original meaning is to tightly seal the top of a flask. It is also used in the meaning of swallowing one's anger since when a person becomes upset, it is said that his anger 'spills out of him.'
At this point, we must explain some issues:

1. Anger is one of the things which leads to sin
The sins that one commits in a state of anger are many, for example: insulting and harming a fellow believer; polluting the tongue with speaking impermissible things, and many other sins. In addition, sometimes a person performs such a sin that he is not able to control himself, or due to his anger, performs something that he would never be able to compensate for during the course of his entire life!
It is for this reason that most of the decisions which are taken in a state of anger are always followed by regret and remorse.
Thus, a person must remind himself that he should only make decisions in his life after coming out of the state of anger since while angry, a person's thoughts are not balanced and do not work properly. In addition, in such a state, one does not have the ability to discern his best interests and it is for this reason that the scholars have stated that, “While in a state of anger, no decision can be made, nothing can be ordered (to be done), nor can any punishment given.” In other words, in such a mental state, no important action can be undertaken.
Rather, what is important is that a person does not make any decision and in this time period, tries to control himself. Of course, to control ones' self while in a state of anger is also difficult to do, however one of the things which is beneficial is to bring about the agents within the self which would result in a change in the mental state.
For example, the person should leave that area where he is, in order to control his emotions; if he has become upset while inside his house, then he should step outside. It has even been stated that if a person is standing then he should sit down or if he is sitting down then he should stand up or even drink some water to change his mood and control himself!
Thus in whatever way possible, one must change the state one is in and even if one is able to change the state for a brief period of time, this too would be beneficial. To accomplish this, one could for example, sprinkle some water on one's face or recite a Surah from the Qur`an.
Sometimes a person goes into such a fit of anger that in order for it to subside, he has no other choice but to punch the wall so as to remove the pressure building up in him, thus preventing his anger from manifesting itself on others!
Truthfully, anger is a flame from the fire of Satan and is one of the most important tools which he makes use of.
The key point to note is that anger takes a person out of his natural state of mind. We have seen people who, while in their 'regular mental state', are very good and down to earth people. However, when they enter into a state of anger, then everything around them begins to fly around the house (whatever they see they begin to throw
In this state, they can not be considered as a normal human being since many changes come about within them. The most important thing which can be stated in regards to the preson in such a stae is that he does not have any religious excuse which he can use such that he could, for example, say that “I was in a state of anger and thus I did so and so…”
We also see that, perhaps a majority of people murdered, are killed when the killer was in a fit of anger and rage since no one would take a knife and slit someone's throat when he is cool and collected!

2. Anger is one of the most dangerous tools as Satan is able to use it to make people commit sins
Anger is one of the gifts from Allah (SwT) which can also be used in incorrect and inappropriate places. As we know, the power of anger actually refers to the gathering of a person's physical energy in the face of a difficulty. Therefore, Allah (SwT) is the one who has created the power of anger so that a person would be able to defend himself from an attack, as if a person was to remain indifferent in such situations and did not gather his inner-energy, then he would not be able to defend himself.
There are some people who have stated that when a person is in his regular state, he has approximately 1/10th of his power ready to use. However, when he gets into a state of anger, then his power becomes ten fold - meaning that all of their energy is mustered!
With this point in mind, we see that in a regular state, some people are very relaxed, however when they enter into a stage of anger, then even five people would not be able to take charge over them and control them as all of their inner energy and power has been gathered together!
Therefore, the power of anger which Allah (SwT) has created within everyone is actually one of His blessings, however it must be used in the proper way.
With this said, we understand that anger is of two types: the unbefitting form of anger and the praiseworthy and obligatory anger.
In the face of those who openly perform sins and transgress the laws of Allah (SwT), the true believer must show his Islamic zeal and enthusiasm in opposing the impermissible acts and must not be indifferent to sins happening around him.
There is a well-known tradition which states that once, Allah (SwT) sent a group of Angels to destroy a particular nation, however they did not fulfill the responsibility given to them by Allah (SwT) and returned back, appealing to Him, saying: “O' Allah! Within this group (whom we are commanded to destroy) we saw a very old man who worships You in the darkness of the night. He was awake in Your worship and we did not want to destroy this nation since this man was amongst them.”
It was revealed to this group of Angels that, “Return back to that nation and destroy the entire community as well as the old man!” Once they were given this command, they were told the reason why this was to be done:
لَمْ يَحْمَرَّ وَجْهُهُ لِغَضَبِ قَطُّ
“The same old man you see awake in the dark hours of the night in worship is the same person whose face never turned red in anger at seeing the sins being committed around him!”
Therefore, when a person must command others to righteous deeds and forbid them from the evil (Amr bil Ma’ruf and Nahi ‘anil Munkar), become angry at sins being committed, leaving that area where sins are being performed and in summary, fulfilling his responsibility in these regards, he must do this.
Again, it is this form of anger which is praiseworthy and is actually obligatory!
From the point of view of the faith of Islam, the person who is indifferent to sins and sinning has absolutely no value!
Of course it goes without saying that anger which is not warranted is something which is not liked by Allah (SwT).

3. The Reward for those who swallow their Anger
a) The first reward granted to those who swallow their anger is that they shall have protection on the Day of Resurrection.
If Allah (SwT) was to grant a person protection on the Day of Resurrection, which is also the day known as: The Day of Grief; The Day of Everything in the Open; The Day of Trepidation; The Day of the Great Fear; and The Day of the Distress, then is there any greater blessing or gift?!
In the same way, if a person was to possess the entire transient world, however did not possess tranquility and peace in his heart, then all of his material possessions would mean nothing! Thus, if one has peace of heart and mind, however has nothing from the material aspect of this world, then in reality, he has everything!
b) The second reward is that such a person shall be granted true faith in Allah (SwT) while in this world.
It has been mentioned that if a person was to refrain from certain sins, then Allah (SwT) would grant that person true faith that he could “taste” - this means that his faith would not be something dry and monotonous in theory. Rather, there would be a sweetness in his faith which he could actually sense! This is not something granted to all people - it is only bestowed upon those who stand firm in the face of their lower desires.
In the introductory chaper on 'Looking at others' in the compilation of traditions, Wasa`il ash-Shi’a, we have traditions which state if a person's eyes fall upon a beautiful woman who is not his mahram and he immediately turns his eyes away from her, then Allah (SwT) would grant that person such a level of faith that the person would actually be able to taste it (the true faith)!

Anger in the Islamic Narrations
1. Imam Ja’far b. Muhammad as-Sadiq (as) has stated that “Anger spoils faith just as vinegar spoils honey.”[2]

قَالَ الإِمَامُ جَعْفَرُ بْنُ مُحَمَّدٍ الصَّادِقُ أَلْغَضَبُ يُفْسِدُ الإِيـمَانَ كَمَا يُفْسِدُ الْخَلُّ الْعَسَلَ

2. Imam ‘Ali b. Abi Talib (as) has said, “I advise you in regards to anger since the beginning stage of it is insanity while the final stage of it is remorse.”

قَالَ عَلِيُّ بْنُ أَبِي طَالِبٍ: إِيَّاكَ وَالْغَضَبَ، فَأَوَّلُهُ جُنُونٌ وَآخِرُهُ نَدَمٌ

3. The Prophet of Islam (S) has said, “The strong person is not the one who can lift heavy weights. Rather, the strong person is the one who is in control of his soul while in a state of anger.”[3]

قَالَ رَسُولُ اللٌّهُ: لَيْسَ الشَّدِيدُ بِالصُّرْعَةِ، إِنَّما الشَّدِيدُ الَّذِي يَمْلِكُ نَفْسَهُ عِنْدَ الْغَضَبِ

4. Imam ‘Ali b. Abi Talib (as) has said, “Anger is a fire which is burning away. The one who is able to swallow it is the one who has extinguished it, while the person who permits his anger to be free is the first person who shall be burnt by it.”[4]

قَالَ الإِمَامُ عَلِيُّ بْنُ أَبِي طَالِبٍ: أَلْغَضَبُ نَارٌ مُوقَدَةٌ. مَنْ كَظَمَهُ أَطْفَأْهَا، وَ مَنْ أَطْلَقَهُ كَانَ أَوَّلَ مُحْتَرَقٌ بِهَا

5. Imam Ja’far b. Muhammad as-Sadiq (as): “Anger is the key to all evil.”[5]

قَالَ الإِمَامُ جَعْفَرُ بْنُ مُحَمَّدٍ الصَّادِقُ: أَلْغَضَبُ مِفْتَاحُ كُلِّ شَرٍّ

6. Imam ‘Ali b. Abi Talib (as) has said, “A strong feeling of anger changes the way a person speaks; destroys the pillars of logic and makes a person's comprehension scattered and disturbed.”[6]

قَالَ الإِمَامُ عَلِيُّ بْنُ أَبِي طَالِبِ: شِدَّةُ الْغَضَبِ تُغَيِّرُ الْمَنْطِقَ، وَ تَقْطَعُ مَادَّةَ الْحُجَّةِ، وَتُفَرِّقُ الْفَهْمَ

7. Imam ‘Ali b. Abi Talib (as) has said, “The staunchest of enemies for a person are his anger and lower desires. The person who is able to take control of these two shall have the greatest rank and shall be able to reach to the pinnacles of humanity.”[7]

قَالَ الإِمَامُ عَلِيُّ بْنُ أَبِي طَالِبٍ: أَعْدى عَدُوٍّ لِلْمَرْءِ غَضَـبُهُ وَشَهْوَتُهُ، فَمَنْ مَلَكَهُمَا عَظُمَتْ دَرَجَتُهُ وَبَلَتْ غَايَـتُهُ

8. The Prophet of Allah (S) has said, “Anytime you get angry, keep quiet.”[8]

قَالَ رَسُولُ اللٌّهُ: إِذَا غَضِبْتَ فَاسْكُتْ

9. Imam Musa b. Ja’far al-Kadhim (as) said, “The one who controls his anger over the people, Allah shall control His anger over that person on the Day of Judgement.”[9]

قَالَ الإِمَامُ مُوسَـى بْنِ جَعْفَرٍ أَلْكَــاظِمُ: مَنْ كَفَّ غَضَـبَهُ عَنِ النَّاسِ، كَفَّ اللٌّهُ عَنْهُ غَضَبَهُ يَوْمَ الْقِيَامَةِ

10. The Noble Prophet (S) has said, “Anytime one of you gets angery, he should wash his face with cold water since surely anger is from the hell fire.”[10]

قَالَ رَسُولُ اللٌّهِ: إِذَا غَضِبَ أَحَدُكُمْ، فَيَتَوَضَّأْ بِالْمَآءِ الْبَارِدِ، فَإِنَّ الْغَضَبَ مِنَ النَّارِ

Notes:
[1] Bihar al-Anwar, vol. 74, pg. 46
[2] Ibid., vol. 73, pg. 267
[3] Majmuatul Warram, vol. 2, pg. 122
[4] Ghurur al-hikm, pg. 292
[5] Ibid., pg. 49
[6] Safinat al-Bihar, vol., pg. 220
[7] Ghurur al-hikm, pg. 498
[8] al-Mahajjat al-Baydaaf1, vol. 5, pg. 308
[9] Wasail al-Shia, vol. 11, pg. 298
[10] al-Mahajjat al-Baydaaf1, vol. 5, pg. 305