Who is the extremist in religion? And by what is he known? And peace be upon you...

The word "extremists" (al-mutashaddidun) has two applications: The first refers to those who are overzealous (al-mutanatti'un), who go to extremes in worship and believe that what the Messenger brought is insufficient for salvation. This is what the Prophet warned against when he said: "Destroyed are those who are overzealous." The second refers to "fundamentalists" (al-usuliyyun).