When Abdullah bin Abbas was the Governor of Basra, Imam Ali (a) wrote the following letter to him. The cause of this letter was the behaviour of Ibn Abbas towards the clan of Bani Tamim. Ibn Abbas hated them because some of them had sided with Talha and Zubayr in the Battle of Jamal and therefore, he had on occasions treated them scornfully. They reported this matter to Imam Ali (a) requesting that the whole clan should not be treated badly because of the folly of a few. This letter shows what a kind rule it was that Imam Ali (a) wanted to introduce.
Understand very well Ibn Abbas that Basra is a satanic place. It is an abode of strifes and bloodshed. So be kind and tolerant towards the citizens of Basra. Win them over with kindness, sympathy and sincerity. Remove fear, suspicion, distrust and animosity from their minds. I am given to understand that you have ill-treated the clan of Bani Tamim and have insulted them.
Remember that Bani Tamim is such a clan that their star has not set as yet, amongst them if one great man dies there is another to take his place. Remember that after embracing Islam and even during pre-Islamic days these people were never regarded as mean, jealous or covetous. On the contrary, they had a very high status. Besides they have claims of kinship and friendship with us. If we behave kindly, patiently and sympathetically towards them Allah will reward us. But if we ill-treat them we shall be sinning.
May Allah have mercy upon you, Ibn Abbas! Be careful about your behaviour towards those over whom you are ruling, be kind to all and be careful about your tongue and your behaviour because you are ruling there on my behalf and your actions are those of mine and I am responsible for them. I have a good opinion about you, please try to be such that I may not be forced to change it.