Sayyiduna Arfajah al-Ashja'i (may Allah be pleased with him) said: I heard the Prophet (peace and blessings be upon him) saying: "After me there will be trials and troubles" - he (peace and blessings be upon him) drew out this word as he said it - "so whoever 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 narrator) swore that he (peace and blessings be upon him) did not make an exception for anyone among the people.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14429