Al-Imam Abu Haneefah (may Allah have mercy upon him) stated:

“It is befitting for the Imam (the Muslim ruler), when the news reaches him that the Khawaarij are preparing for combat, to seize them and imprison them in order that they leave this off. This is due to the fact that repelling the evil before it takes place is easier than repelling it after it has happened.”

Source: Mueen al-Hukkaam Feemaa Yataraddidu Bayna al-Khasamayn min al-Ahkaam 2/427
Translated by: Abu Yusuf Khaleefah
11th of Dhul Hijjah, 1435 (5th of October, 2014)
Masjid Nur Allah, LeFrak City, Corona, NYC