It is narrated from Arfajah - who was a man from the narrator's people - that the Messenger of Allah (peace and blessings be upon him) said: "Whoever advances towards my Ummah, while they are united, with the intention of sowing division among them, kill him, whoever he may be."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14432