Abdullah bin Qays, better known in history as Abu Musa Ash'ari, was a man with weak faith, more inclined to look after his worldly interest than the cause of religion. At the beginning of the Caliphate of Imam Ali (a) he was in Kufa. When he learnt that Talha and Zubayr along with Ummul Mu'minin, Aisha, had revolted against the Caliphate of Imam Ali (a) and had declared a war against him and had made Basra as their Headquarters and Imam Ali (a) had also started mobilising an army against them and had invited the people of Kufa to join that army, after realizing that the party of Talha and Zubayr was also a very rich and influential group and that it would be a wise action to be friendly with both the parties, he started saying, “Though Ali was the lawful caliph of Muslims, yet it was not correct for him to fight against other Muslims”. Imam Ali (a) came to know that Abu Musa though professing his faith in him, was yet persuading people not to help him (Imam Ali (a)). Thereupon, he sent the following letter to Abu Musa through Imam Hasan (a).
From the creature of Allah, Ali bin Abi Taalib to Abdullah bin Qays (Abu Musa Ash'ari).
After glorifying Allah and praising the Holy Prophet (s) let it be known to you that I have received reports which may be considered favourable to you, and yet at the same time may be deemed against you (that you on one hand declare me to be a lawful caliph and in the same breath you persuade others not to come to my help). As soon as my messenger reaches you, get ready, come out of your retreat, invite your people to join me and come yourself. If you are convinced that I am on the right side, then you must come to my help and if you feel uncertain, nervous or timid, then get you gone.
I swear by Allah that I shall not let you waver or adopt a double-cross policy. I shall not allow you to sit peacefully at home with a double face, one for each party and I shall expose you to the people. You will find yourself under suspicion from either group and you will be forced to declare your true inclinations.
This revolt which has made Basra as its Headquarters is not an easy joke as you have imagined it to be. It is a great tragedy and it shall have very far-reaching effects. It has to be faced with the calamities which accompany it or which will follow it. It will have to be fought and to get the better of. Therefore, I advise you to think carefully, control your weak and wavering mind and face your lot bravely. And if you are not satisfied with me or have no faith in me then you can with complete freedom go to the other side. You have my unreserved permission. But I am sure you will not be welcomed there.
And if you remain in my camp I shall not allow you to enjoy a restful sleep when Islam itself is in troubles and at war with the rebels. I swear by Allah that this decision of mine is the right step of a faithful Muslim in the right direction.