Benefit: Fighting against the Khawārij Is Rewarded!
In the Name of Allāh, the Ever Merciful, the Bestower of Mercy
About whom was this narration mentioned about? And which time frame was the Prophet (صلى الله عليه وسلم) indicating?
Answer: This narration and that which is in it was said by the Prophet (صلى الله عليه وسلم) about a sect who are called 'al-Khawārij' due to their extremism in the religion and their expelling of Muslims (from Islām) because of sins which do not take someone out of Islām. They also rebelled against ʿAlī ibn Abī Ṭālib (may Allāh be pleased with him) and they refuted him on particular issues, so he (ʿAlī ibn Abī Ṭālib) called them to the truth and debated them. Many of them returned to the truth and some of them remained (upon their false ideology). So when the aggression against the Muslims past its limit, ʿAlī ibn Abī Ṭālib (may Allāh be pleased with him) fought them and the Muslim leaders who came after him fought them as well, in accordance with the narration mentioned (in the question) and its meaning. There are remnants (of the Khawārij) till this very day, and the ruling upon them is general to everyone who holds their (Khārijī) beliefs in every time and place.
The Permanent Committee for Islamic Research and Fatāwá
Translator's note: Without a doubt, a crucial method of fighting against the Khawārij (terrorist, renegades) is by speaking and writing against them at any and every given muʿmīnt of opportunity, as our Scholars and their scholars and so on have been doing for centuries. There are many ways you could help your community, by collaborating with them in handing out material on the subject matter of terrorism and its ill effects or simply by guiding them to those who may help in educating them about the true Islamic beliefs with regards to the khawārij.
Source: al-Lajna al-dāʾimah, Juz #2, pg. 369
Translated by: Munīb al-Ṣumālī