Amirul Momineen (s) really considered Umar unjust and deviated from the Sunnah of Prophet (s.a.w.s.).

The books of Ahle Sunnat are replete with such discussions. Allamah Qutaybah writes in Al-Imamah Was-siyaasah about the battle of Naharwan: Amirul Momineen (s) asked a Khatha’mi man to pay allegiance. He said that he would pay allegiance to him on the command of Quran, Sunnah of Prophet (s.a.w.s.) and method of Shaykhs. Amirul Momineen (s) asked, “Why do you mix the method of Abu Bakr and Umar with the Book of God and Sunnah of Prophet (s.a.w.s.)? The two were tyrannical and unjust rulers.”

In the same way the following tradition of Amirul Momineen (s) is quoted on Pg. 154 of Tanqeed-e-Akhbar on the authority of Kanzul Ummal, Kitabul Iktiqa and Tabaqat of Ibne Saad:

Amirul Momineen Ali (s) said, “So I came to know, that by Allah, Umar would surely not act on justice according to the Sunnah of Prophet (s.a.w.s.).”

That is why Amirul Momineen (s) never considered the two Shaykhs just and rightful. So he did not accept the condition of the Shaykhs’ practice at the time of consultation (Shura) and spurned the apparent caliphate. These thoughts of Ali were not such that they be hidden and only revealed after the death of he Shaykhs but they were obvious in their lifetime also. Even caliph Umar was aware of it. Thus, according to one of the narrations of Sahih Muslim (belief on whose traditions is one of the necessities of religion of Ahle Sunnat), Umar himself complained inter alia a long discussion with Amirul Momineen (s) and Abbas bin Abdul Muttalib and said, “You both considered me a liar, sinner, disloyal and treacherous.”

Obviously, when other proven sayings of Amirul Momineen (s) show that he considered Abu Bakr and Umar unjust, deviated from the way of the Prophet (s.a.w.s.), liar, sinner, disloyal and treacherous, hence if a saying of Amirul Momineen were presented whose meaning is somewhat similar to this we would surely have to believe in that meaning only but it would be necessary to reject the meaning, which is opposite to these sayings.