Tirmidhi

Explanation:
1:
In this hadith, there is testimony from the tongue of Allah’s Messenger (sallallahu alayhi wa sallam) that at the time of his killing, Ammar (radi Allahu anhu) would be with the group upon the truth, and his killers, in this regard, would not be upon the truth.
And by “truth and falsehood” here, what is meant is not the truth and falsehood of creed (i.e., tawhid and shirk), but rather, it is a testimony regarding being upon truth or falsehood in a political sense at that time.
Ammar (radi Allahu anhu) at that time was with Ali (radi Allahu anhu), who was the rightful caliph at that time,
and the opposing party was Muawiyah (radi Allahu anhu), who was fighting Ali (radi Allahu anhu) over a political issue concerning the caliphate.
It was in this context that Ammar (radi Allahu anhu) was martyred.
This is the incident of the Battle of Siffin. In this incident, Muawiyah (radi Allahu anhu), who had come to oppose Ali (radi Allahu anhu),
this was his ijtihadi (independent juristic) error, which is forgiven,
(radi Allahu anhum ajma‘in).