‘Arfajah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If someone comes to you seeking to undermine your solidarity or disrupt your unity when you have agreed on a man (as your leader), kill him."
شرح الحديث :
This Hadīth is about fighting rebels who want to divide the Muslims after they have unified and agreed on a ruler. It also indicates that if the Muslims have agreed to appoint a specific caliph or ruler and someone wants to depose the man they have agreed on, it is incumbent upon them to resist this person, even if that results in killing him, in order to avert the harm and prevent the bloodshed of Muslims.