Sahih Muslim Book 020, Hadith Number 4565.
Chapter : Decision about one who tries to disrupt the unity of the Muslims.
It has been narrated on the authority of 'Arfaja who said: I have heard the Messenger of Allah (may peace be upon him) say: Different evils will make their appearance in the near future. Anyone who tries to disrupt the affairs of this Umma while they are united you should strike him with the sword whoever he be. (If remonstrance does not prevail with him and he does not desist from his disruptive activities, he is to be killed.)