Sayyiduna Arfajah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "Soon there will be trials and troubles. So whoever comes to the Ummah of Muhammad (peace and blessings be upon him) while they are united, intending to divide them, kill him, whoever he may be."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14425