Now, Allah the Glorified, deputed Muhammad (may Allah bless him and his descendants) as a warner for all the worlds and a witness for all the prophets. When the Prophet expired, the Muslims quarrelled about power after him. By Allah, it never occurred to me, and I never imagined, that after the Prophet the Arabs would snatch away the caliphate from his Ahlul Bayt (the members of his house), nor that they would take it away from me after him, but I suddenly noticed people surrounding the man to swear him allegiance. 1
I therefore withheld my hand till I saw that many people were reverting from Islam and trying to destroy the religion of Muhammad (may Allah bless him and his descendants). I then feared that if I did not protect Islam and its people and there occurred in it a breach or destruction, it would mean a greater blow to me than the loss of power over you which was, in any case, to last for a few days of which everything would pass away as the mirage passes away, or as the cloud scuds away. Therefore, in these happenings I rose till wrong was destroyed and disappeared, and religion attained peace and safety.
A part of the same letter
By Allah, if I had encountered them alone and they had been so numerous as to fill the earth to the brim, I would not have worried or become perplexed. I am clear in myself and possess conviction from Allah about their misguidance and my guidance. I am hopeful and expectant that I will meet Allah and get His good reward. 
But I am worried that foolish and wicked people will control the affairs of the entire community, with the result that they will grab the funds of Allah as their own property and make His people slaves, 2 fight with the virtuous, and ally with the sinful. Indeed, there is among them he who drank (wine) unlawfully 3 and was whipped by way of punishment fixed by Islam, and there is he who did not accept Islam until he had secured financial gain through it. 4 
If this had not been so I would not have insisted on gathering you, reprehending you, mobilizing you and urging you (for jihad) but if you refuse and show weakness I will leave you.
Do you not see that the boundaries of your cities have diminished, your populated areas have been conquered, your possessions have been snatched away and your cities and lands have been attacked? May Allah have mercy on you, get up to fight your enemy and do not remain confined to the earth, otherwise you will face oppression and suffer ignominy and your fate will be the worst. The warrior should be wakeful because if he sleeps the enemy does not sleep; and that is an end to the matter.