The following is a tradition of al-‘Imam al-Sadiq (A) from the noble al-Kafi, in which the Imam says:
Verily, sabr is to faith what the head is to the body. The body perishes without the head, and so also when sabr goes faith also disappears? 
In another tradition whose isnad reaches al-‘Imam al-Sajjad (A), he is reported to have stated:
Verily, sabr is to faith what the head is to the body: one who has no sabr has no faith .
There are many traditions relating to this topic and we will mention some of them later in relevance with the context.
Sabr is the key to the gates of felicity and the main means of deliverance from mortal perils. Sabr makes man bear calamities with ease and face difficulties with composure. It strengthens the will and the power of resolution. It brings independence to the dominion of the soul. Anguish and anxiety on the other hand, aside from their shameful character, are symptoms of the soul’s weakness. They deprive one’s being of its stability, weaken the determination, and enfeeble the intellect. The informed researcher Khwajah Naqir al-Din al-Tusi – may God sanctify his soul – states:
Sabr restrains the inner being from anguish, the tongue from complaint and the bodily members from untoward movements.
On the contrary, the inward being of the impatient person is full of anxiety and alarm. His heart is full of tremors and shaky, and this. is itself the greatest calamity that can befall a human being and deprive him of peace. But sabr diminishes misfortune and makes the heart overcome hardship and assists the will to overpower calamities. In the same way, the impatient person complains of his troubles to everyone, and this, aside from causing disrepute among people who look down upon him as a weakling of instable character, makes him lose his standing in the court of the Holy Lord and in front of His angels. What faith in
God and what surrender to the Holy Lord has the servant who cannot bear an adversity that visits him from his Beloved, the Absolute, after having received thousands upon thousands of His bounties and while being immersed in the sea of His favour, and opens his mouth in front of people to complain as soon as an adversity strikes him? So it is right if it is said that one who has no sabr has no faith.
If you have faith in your Lord and believe all the affairs to lie in His mighty hands, if you believe none else to have any say in the matters, of course you will not complain of the hardships of life and the adversities that strike you before God Almighty. Rather you will accept them willingly and thank Him gratefully for His bounties. Hence that inner agitation, those plaintive
utterances, those unbecoming bodily movements – all these bear testimony to our lack of faith. As long as we find ourselves amid favours, we are grateful, with a gratefulness that is formal and lacking inner substance, a gratefulness that is offered in greed of further favours. However, when a tragedy occurs or a pain or disease strikes us, we take our complaints against the Blessed and the Almighty Lord in front of the creatures. With a plaintive tongue and a tone brimming with taunts and cynicism, we complain about Him to all and sundry. Gradually those complaints, anxieties and misgivings sow the seeds of hostility towards God and His decrees inside the self. Slowly they sprout and grow, making the once passing feeling into an enduring trait. Thereupon, God forbid, the form of one’s inward being assumes the form of enmity towards God and His ordainments. When that happens, things go out of one’s control and one is no longer able to check his thoughts and feelings. His inward and outward being becomes coloured with the hostility towards God Almighty, and he departs from this world to face eternal wretchedness and gloom, with a spirit infused with the enmity and hatred of the Lord of Bounties. I seek refuge in God from the evil of a disastrous end and from a provisional faith (iman mustawda`). Hence it has been truly said that when sabr goes faith also departs.
And so, my dear, the matter is of great importance and the road is full of perils. Muster all the strength at your command and be patient and forbearing throughout the ups and downs of life. With manly courage stand against adversities and calamities. Make your self understand that agitation and anguish, aside from involving a great shame by themselves, are futile in confrontation with sufferings and calamities. No purpose is served by complaining about the irresistible decrees and inevitable ordainments of God in front of weak and powerless creatures, as pointed out by the following noble tradition from al-Kafi:
Muhammad ibn Ya’qub al-Kulayni reports with his chain of transmitters from Sama’ah ibn Mihran from al-‘Imam al-Kazim (A) that the Imam said to him, "What stopped you from going for hajj?" Sama’ah says that he replied, "May I be your ransom, a big debt has fallen upon me and I have lost my property. However, the debt that lies upon me weighed more heavily upon me than the loss of property. Had it not been for one of our companions, I could not have come out of it." The Imam said, "If you are patient, you will be the object of envy, and if you don’t God will enforce His decrees regardless of your pleasure or displeasure."
Thus we know that anxiety and anguish are not only futile, they are capable of inflicting terrible injuries and are followed by fatal consequences destructive to faith. On the other hand, sabr, forbearance and restraint bring fair, plentiful rewards and have sublime and beautiful forms in the world of Barzakh. This is stated at the end of the noble tradition expounded by us:
Thus the ultimate result of qabr is good in this world – as known from the example of Hadrat Yusuf (A) – and it is the cause of reward in the Hereafter. In another noble tradition of al-Kafi, whose sanad goes up to Abu Hamzah al-Thumali – may God’s mercy be upon him -al-‘Imam al-Sadiq (A) is reported to have said:
Abu Hamzah says: "Abu `Abd Allah said: `Whoever of the believers that bears patiently with a tribulation that befalls him, has the reward of a thousand martyrs.’ " 
There are many traditions relating to this subject and we will mention some of them in the following section. However, as to that which was said above that sabr has a beautiful purgatorial form, this is mentioned – apart from the demonstrative proofs confirming it – in the following tradition of the noble al-Kafi from al-‘Imam al-Sadiq (A):
The Imam (A) said: "When the believer enters his grave, salat is on his right hand, zakat on his left, virtue faces him, with sabr taking him under its shelter. When the two angels encharged with questioning enter upon him, sabr says to salat, zakat and virtue, `Take care of your companion, and if you fail to assist him I will take care of him myself.’ " 
The Degrees and Levels of Sabr:
Let it be known to you that, as indicated by the noble traditions, there are various degrees and levels of sabr, and its reward and merit vary in accordance with its degree and level. This is revealed by the following tradition of the noble al-Kafi narrated by the Master of the God-fearing, Amir al-Mu’minin ‘Ali (A) from the Holy Prophet (S):
‘Ali (A) said, "The Messenger of Allah said, ‘Sabr Is of three kinds: sabr at the time of affliction, sabr in regard to obedience, and sabr In regard to disobedience. One who bears patiently with affliction, resisting it with a fair consolation, God writes for him three hundred degrees (of sublimity), the elevation of one degree over another being like the distance between earth and heavens. And one who is patient in regard to obedience, God writes for him six hundred degrees (of sublimity), the elevation of one degree over another being like the distance between the earth’s depths and the Throne (al-‘arsh). And one
who is patient in regard to disobedience, God writes for him nine hundred degrees (of sublimity), the elevation of one degree over another being like the distance between the earth’s depths and the furthest frontiers of the Throne: "
This noble tradition reveals that sabr in regard to disobedience is superior to the other levels of sabr, for it not only possesses greater number of degrees but also the range between its degrees is greater than those of the other kinds. It also shows that the extent of Paradise is much greater than what can come into our imagination, for our vision is limited and confined. That which has been said as a description of Paradise that:
…And a Garden the breadth whereof is as the breadth of heaven and earth …. (57:21)
perhaps refers to the Paradise of deeds. That which has been referred to here in this noble tradition is the Paradise of character and the criterion in the Paradise of character is the strength and perfection of will. Hence its extent should not be confined within any limits. Some have stated that that which has been meant here is height. That is, it is possibly equal in breadth (to the Paradise of deeds) and different in height. But this appears to be remote, for evidently `breadth’ here refers to extent not to breadth as opposed to length, for `breadth’ in reference to heaven and earth also has no meaning when taken in
the ordinary sense of something opposed to length, although they do have `breadth’ in the sense of a `second dimension’ in the terminology of natural philosophers. The Book of God, however, does not speak in accordance with any specific terminology.
The noble al-Kafi records the following tradition of the Prophet (S) with a chain of narration reaching al-‘Imam al-Sadiq (A):
The Imam (A) said: "The Messenger of Allah (S) said, `A time will come upon the people wherein political authority will not be attainable without bloodshed and tyranny, nor wealth without usurpation and stinginess, nor affection without discarding religion and pursuing lust and desire. Whoever lives in such times and bears patiently with poverty despite possessing the capacity to become rich (illegitimately), and bears up with hostility despite being capable of acquiring favour and affection, and bears with humiliation despite possessing the capacity to acquire honour, God will give him the reward of fifty truthful ones, of those who confirmed me.’ " 
Something close to this has been said in another tradition from Amir al Mu’minin (A) and there are many traditions relating to this theme. The few which have been quoted here are sufficient for our purpose.
. Usul al-Kafi, ii, bab al-sabr, 128, hadith No. 2.
. Ibid., hadith No. 3.
. Ibid., hadith No. 10.
. Ibid.. hadith No. 17.
. Ibid., hadith No. 8.
. Ibid., hadith No. 15.
. Ibid., hadith No. 12.
Source: Selected excerpt from ‘Forty Hadith’ by Imam Khomeini