It is narrated from Abu Zayd al-Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "They will call to Allah, the Mighty and Majestic, yet they will have no connection with Allah (and His religion). Whoever fights them will be more deserving (of nearness) to Allah than they are." - He (peace and blessings be upon him) meant the Khawarij.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14121