Sayyiduna Arfajah bin Durayh al-Ashja'i (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "After me there will be trials and troubles. So whoever you see who has separated from the community (jama'ah), or who intends to divide the Ummah of Muhammad (peace and blessings be upon him) while their affair is united, kill him, whoever he may be. The Hand of Allah is over the community, and Satan is with the one who separates from the community."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14434