The historian, Ibn Kathīr writes in al-Bidāyah (Dār Ibn Kathīr, 1431H) 8/8 in the events of the year 40H:

The Revolt of a Faction of the Khārijites Against Him: And the reason for that was that when Muʿāwiyah entered Kūfah and al-Ḥasan bin ʿAlī and his family left it to go to the Ḥijāz, a faction of the Khārijites – and they were around 500 in number – said: ‘There has occurred that in which there is no doubt, so proceed to Muʿāwiyah and make jihād against him.’ So they proceeded to Kūfah being presided over by Farwah bin Nawfal. Muʿāwiyah sent soldiers on horseback to them from the people of Syria. But the Khārijites repelled the Syrians. So Muʿāwiyah said to the Kūfans: ‘You have no guarantee of safety from me until you halt your calamities. So they departed towards the Khārijites who said to them: ‘Woe to you! What do you desire? Is not Muʿāwiyah your enemy and our enemy? Leave us alone so that we can fight him. If we defeat him, then we have sufficed you from him. And if he defeats us, you would have sufficed us.’ They said: ‘No by Allāh, not until we fight you.’ The Khārijites said: ‘May Allāh have mercy upon our brethren from the people of the river [alluding to those defeated by ʿAlī at Nahrawān], for they were more knowledgeable about you O people of Kūfah. Then the Kūfans defeated them and expelled them.”

Ibn Kathīr also writes in the events of 43H (8/13-14):

And in [this year] was the great occurrence between the Khārijites and the army of Kūfah. And this is because they had organised and prepared themselves – as we have already mentioned – in revolting against the people in that time.

Then he goes on to discuss the details of the battles that took place between them with the Companion, al-Mughīrah bin Shuʿbah leading the battle against them through the organisation and arrangement of military force. The Khārijites had previously attempted to assassinate Muʿāwiyah, governor of Syria, as well as ʿAmr bin al-ʿĀṣ, governor of Egypt, failing in both whilst succeeding in the assassination of ʿAlī. They did not stop in their war against the seats of power in the Muslim world at the time and continued revolting.

Abu Iyāḍ. 17 Muḥarram 1438 / 18 October 2016