When Muhammad bin Abi Bakr was killed in Egypt by the guerrillas of Mu’awiya through disloyalty of his (Muhammad's) own companions and officers, Imam Ali (a) felt sad and wrote the following letter to Ibn Abbas.
Ibn Abbas! Muhammad, (may his soul rest in peace), died as a martyr and Egypt has fallen in the hands of the enemies. I beseech the Reward of Allah for the sorrow I feel and the grief I suffer on account of the martyrdom of this young man who was like a son to me. He loved me. He was faithful to me. So far as the defense of the Islamic State is concerned he was like a sharp sword and an impregnable fort.
Long before this deplorable event I had issued orders to the Heads of various provinces and to the people of Kufa to reach him or send help to him when he calls for it. I had repeated these orders. Some went to his help but half-heartedly, others started submitting excuses, while some sent false reports of important engagements and did not co-operate with him.
I feel disgusted and pray to Allah to relieve me of the society of such faithless and worthless people.
I swear by Allah that had I no desire of dying as a martyr, and had I not been ready for my death awaiting night and day, I would not have liked to live amongst them even for a day and I would not have come out with them to fight against the enemies of Allah and Islam.