The Shia websites are often filled with the following story about the arbitration after the battle of Siffin:
After the nomination of arbitrators when the deed of agreement was being written, then with `Ali ibn Abi Talib (p.b.u.h.) the word Amir al-mu’minin was also written. `Amr ibn al-`As said, “This should be rubbed off. If we regarded him Amir al-mu’minin why should this battle have been fought?” At first Amir al-mu’minin refused to rub it off but when they did not in any way agree, he rubbed it off and said, “This incident is just similar to the one at al-Hudaybiyah when the unbelievers stuck on the point that the words ‘Prophet of Allah’ with the name of the Prophet should be removed and the Prophet did remove it.” On this `Amr ibn al-`As got angry and said, “Do you treat us as unbelievers?” Amir al-mu’minin said, “On what day have you had anything to do with believers and when have you been their supporters?” However, after this settlement, the people dispersed, and after mutual consultation these two arbitrators decided that by removing both `Ali and Mu`awiyah from the Caliphate the people should be accorded the power to choose whomever they desired. When time came to its announcement there was a meeting at Dumatu’l-Jandal, a place between Iraq and Syria, and then two arbitrators also reached there to announce the judgement on the fate of the Muslims. Acting cunningly `Amr ibn al-`As said to Abu Musa, “I regard it ill manner to precede you. You are older in years and age so first you make the announcement.” Abu Musa succumbed to his flattery and came out proudly and stood before the gathering. Addressing them he said, “O’ Muslims we have jointly settled that `Ali ibn Abi Talib and Mu`awiyah should be removed and the right to choose a Caliph be accorded to the Muslims. They should choose whomever they like.” Saying this he sat down. Now the turn was for `Amr ibn al-`As and he said, “O’ Muslims you have heard that Abu Musa removed `Ali ibn Abi Talib. I also agree with it. As for Mu`awiyah, there is no question of removing him. Therefore I place him in his position.” No sooner that he said this there were cries all round. Abu Musa cried hoarse that it was a trick, a deceit and told `Amr ibn al-`As that, “You have played a trick, and your example is that of a dog on which if you load something he would gasp, or leave him he would gasp.” `Amr ibn al-`As said, “Your example is like the ass on whom books are loaded.” However `Amr ibn al-`As’s trick was effective and Mu`awiyah’s shaking feet were again stabilised. This was the short sketch of the Arbitration whose basis was laid in the Qur’an and sunnah. But was it a verdict of the Qur’an or the result of those deceitful contrivances which people of this world employ to retain their authority? Could these pages of history be made a torch-guide for the future and the Qur’an and sunnah be not used as a means of securing authority or as an instrument of worldly benefits.
Its narrator is Abu Makhnaf, and he is a well known liar, and his narrations are rejected.
قال أبو مخنف حدثني أبو جناب الكلبي أن عمرا وأبا موسى حيث التقيا بدومة الجندل أخذ عمرو يقدم أبا موسى في الكلام يقول إنك صاحب رسول الله صلى الله عليه و سلم وأنت أسن مني فتكلم وأتكلم فكان عمرو قد عود أبا موسى أن يقدمه في كل شيء اغتزى بذلك كله أن يقدمه فيبدأ بخلع علي قال فنظر في أمرهما وما اجتمعا عليه فأراده عمرو على معاوية فأبى وأراده على ابنه فأبى وأراد أبو موسى عمرا على عبدالله بن عمر فأبى عليه فقال له عمرو خبرني ما رأيك قال رأيي أن نخلع هذين الرجلين ونجعل الأمر شورى بين المسلمين فيختار المسلمون لأنفسهم من أحبوا فقال له عمرو فإن الرأي ما رأيت فأقبلا إلى الناس وهم مجتمعون فقال يا أبا موسى أعلمهم بأن رأينا قد اجتمع واتفق فتكلم أبو موسى فقال إن رأيي ورأي عمرو قد اتفق على أمر نرجو أن يصلح الله عز و جل به أمر هذه الأمة فقال عمرو صدق وبر يا أبا موسى تقدم فتكلم فتقدم أبو موسى ليتكلم فقال له ابن عباس ويحك والله إني لأظنه قد خدعك إن كنتما قد اتفقتما على أمر فقدمه فليتكلم بذلك الأمر قبلك ثم تكلم أنت بعده فإن عمرا رجل غادر ولا آمن أن يكون قد أعطاك الرضا فيما بينك وبينه فإذا قمت في الناس خالفك وكان أبو موسى مغفلا فقال له إنا قد اتفقنا فتقدم أبو موسى فحمد الله عز و جل وأثنى عليه ثم قال ايها الناس إنا قد نظرنا في أمر هذه الأمة فلم نر أصلح لأمرها ولا ألم لشعثها من أمر قد أجمع رأيي ورأي عمرو عليه وهو أن نخلع عليا ومعاوية وتستقبل هذه الأمة هذا الأمر فيولوا منهم من أحبوا عليهم وإني قد خلعت عليا ومعاوية فاستقبلوا أمركم وولوا عليكم من رأيتموه لهذا الأمر أهلا ثم تنحى وأقبل عمرو بن العاص فقام مقامه فحمد الله وأثنى عليه وقال إن هذا قد قال ما سمعتم وخلع صاحبه وأنا أخلع صاحبه كما خلعه وأثبت صاحبي معاوية فإنه ولي عثمان بن عفان والطالب دمه وأحق الناس بمقامه فقال أبو موسى مالك لا وفقك الله غدرت وفجرت إنما مثلك كمثل الكلب إن تحمل عليه يلهث أو تتركه يلهث قال عمرو إنما مثلك كمثل الحمار يحمل أسفارا وحمل شريح بن هانئ على عمرو فقنعه بالسوط وحمل على شريح ابن لعمرو فضربه بالسوط وقام الناس فحجزوا بينهم وكان شريح بعد ذلك يقول ما ندمت على شيء ندامتي على ضرب عمرو بالسوط ألا أكون ضربته بالسيف آتيا به الدهر ما أتى والتمس أهل الشأم أبا موسى فركب راحلته ولحق بمكة
(History of Tabari)
And ibn hajar said “Not trustworthy”
Abu Hatim says Matrook
Darqutni says Dhaeef
Yahya bin Mueen says not thiqqah
Ibn Adi said a shiite and narrates their narrations
Tahir patni says liar
Lisan al mizan 4/462, mizan al aitdal 2/320, tazikratul mouzuat p. 286
So you can see, it is basically a narration by a narrator who has been severely condemned by the scholars. It is also against logic, arbitration is never done in this manner.
Rather Masudi says, they didn’t made speech at all, rather they wrote their decision on a sahifa (i.e paper).
Neither was Abu Musa Ashari a fool like the Shiites suppose him to be, Abu Aswad says: “I didn’t saw a greater scholar in Kufa than Ali and Abu Musa”
Tarikh Islam by Dhahabi , 2/357
Tazkiratul Huffaz , 4/106
The reality is that this is actually a lie , and the history books are often filled with such lies, lets read what Tabari wrote in introduction to his book
I shall likewise mention those (narrators) who came after them, giving additional information about them. I do this so that it can be clarified whose transmission (of traditions) is praised and whose information is transmitted, whose transmission is to be rejected and whose transmission is to be disregarded…The reader should know that with respect to all I have mentioned and made it a condition to set down in this book of mine, I rely upon traditions and reports which have been transmitted and which I attribute to their transmitters. I rely only very rarely upon (my own) rationality and internal thought processes. For no knowledge of the history of men of the past and of recent men and events is attainable by those who were not able to observe them and did not live in their time, except through information and transmission produced by informants and transmitters. This knowledge cannot be brought out by reason or produced by internal thought processes. This book of mine may contain some information mentioned by me on the authority of certain men of the past, which the reader may disapprove of and the listener may find detestable, because he can find nothing sound and no real meaning in it. In such cases, he should know that it is not my fault that such information comes to him, but the fault of someone who transmitted it to me. I have merely reported it as it was reported to me.
(Tareekh at-Tabari, Vol.1, Introduction)
And we can neither expect such foolishness from a grand scholar, and nor such cursing as have been attributed to them.