From the slave of Allah, 'Ali, Amir al-mu'minin to the people who became wrathful for the sake of Allah when He was disobeyed on His earth and His rights were ignored and oppression had spread its coverings over the virtuous as well as the vicious, on the local as well as the foreigner. Consequently, no good was acted upon nor any evil was avoided.
Now, I have sent to you a man from among the servants of Allah who allows himself no sleep in days of danger, nor does he shrink from the enemy at critical moments. He is severer on the wicked than a blazing fire. He is Malik ibn al-Harith, our brother from (the tribe of) Madhhij. 
Therefore, listen to him and obey his orders that accord with right, because he is a sword among the swords of Allah, whose edge is not dull and which does not miss its victim. If he orders you to advance, advance, and if he orders you to stay, stay, because he surely neither advances or attacks nor puts anyone backward or forward save with my command. I have preferred him for you rather than for myself because of his being your well-wisher and (because of) the severity of his harshness over your enemies.