Sayyiduna Arfajah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "After me there will be trials and troubles. So whoever you see intending to divide the Ummah of Muhammad (peace and blessings be upon him) while they are united, kill him, whoever he may be."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14427