What is the authenticity of attributing to Ibn Umar the saying: "I do not regret anything except my not fighting the rebellious group alongside Ali," and his saying: "I withheld my hand and did not step forward, and the one who fights for the truth is superior"? Is it obligatory to support the truth for whoever recognizes it, or is isolation permissible, with a commentary on what Al-Nawawi said in his commentary on Muslim?

What transpired among the Companions, whether fighting or refraining from it, was due to their individual ijtihad (independent reasoning). One should refrain from delving into their disputes, and instead love them, praise them, and not disparage them, while acknowledging their virtue and the fact that they were not infallible. Their errors, born of ijtihad, are forgiven. A Muslim should busy himself with propagating the virtues and merits of the Companions, and refrain from searching for their mistakes.

As for Ibn Umar's regret for not fighting alongside Ali, it has been narrated by a number of imams. Some scholars believe that what is meant is fighting Mu'awiya, and that this is the correct view, while Ibn Taymiyyah believes that what is meant is fighting the Khawarij.