Arfajah bin Shuraih (RAA) narrated, 'I heard the Messenger of Allah (ﷺ) say, "He who comes to you when you are united and wants to disunite your community, kill him." Related by Muslim. (DE)
English reference (DE): Book 9, Hadith 1236 (DE)
Arabic reference (DE): Book 9, Hadith 1208 (DE)