Is Mu'awiyah, may Allah be pleased with him, considered a Nasibi because he was among those who fought in Siffin, and is it permissible to believe that he was a transgressor based on the Hadith, "Woe to Ammar, he will be killed by the transgressing group," while affirming his Islam and faith?

Al-Dhahabi differentiated between the beliefs of the Shi'a and the Nawasib in the early Islamic period and their beliefs in his own time. In the early period, the Shi'a did not declare Mu'awiya and his party to be disbelievers, nor did the Nawasib declare Ali and his party to be disbelievers. However, the Shi'a of his time declared the Companions to be disbelievers, whereas the Nawasib of his time were few. The disagreement between Mu'awiya and Ali, may Allah be pleased with them both, was not over Ali's right to the caliphate, but rather over whether to prioritize immediate retribution for Uthman's killers or to delay it. Mu'awiya himself acknowledged Ali's superiority and his right to leadership. The hadith "Woe to Ammar, he will be killed by the transgressing group (al-fi'a al-baghiya)" does not imply sinfulness for those who killed Ammar among the striving interpreters (mujtahidin) who erred. For an erring mujtahid, his mistake is forgiven, and he receives a reward. The rebellious interpreter (al-baghi al-muta'awwil) who believes himself to be on the right path does not incur sin merely by being called a transgressor. Fighting transgressors (bughat) is to repel the harm of their transgression, not to punish them. All the Companions are people of Paradise, and even if they had sins, Allah forgives them for many reasons. Believing that Mu'awiya erred in his ijtihad and calling his army transgressors does not necessitate judging them to be sinful. Indeed, Ali, may Allah be pleased with him, considered those who fought him from Mu'awiya's companions to be believers.