How many rulers are the Muslims allowed to have?

Abu Sa’id al-Khudri narrated that the Prophet (saw) said: “When the oath of allegiance has been taken for two Khalifahs, kill the latter of them”. [Muslim]

Al-Imam Al-Nawawi in his book Mughni Al-Muhtaj, volume 4, page 132 says: “It is forbidden to give an oath to two Imams or more, even in different parts of the world and even if they are far apart”.

Al-Imam Ibnu Hazm in his book Al-Muhalla, volume 9, page 360 says, “It is permitted to have only one Imam in the whole of the world.”

Al-Imam Al-Sha’rani in his book Al-Mizan, volume 2, page 157 says: “It is forbidden for Muslims to have in the whole world and at the same time two Imams whether in agreement or discord.”

Al-Imam Al Qalqashandi in his book Subul Al-Asha, volume 9, page 277 says, “It is forbidden to appoint two Imams at the same time”.

Abdullah b. ‘Amru b. al-‘A’as said that he heard the Messenger of Allah (saw) say: “Whoever pledged allegiance to an Imam giving him the clasp of his hand and the fruit of his heart, he should obey him as long as he can, and if another comes to dispute with him, you must strike the neck of the latter”. [Muslim]

Afrajah said: I heard the Messenger of Allah (saw) say: “Whosoever comes to you while your affairs has been united under one man, intending to break your strength or dissolve your unity, kill him.” [Muslim]

It has additionally been reported in “as-Sirah” of Ibnu Ishaq that Abu Bakr went on to say on the day of Thaqifa: “It is forbidden for Muslims to have two Amirs for this would cause differences in their affairs and concepts, their unity would be divided and disputes would break out amongst them. The Sunnah would then be abandoned, the bida’a (innovations) would spread and Fitna would grow, and that is in no one’s interests”.