‏الإمارة : Hadeeth No:4568 It has been narrated on the authority of Aba Sa'id al-Khudri that the Messenger of Allaah (sallAllaahu alayhi wa sallam) said: When oath of allegiance has been taken for two caliphs, kill the one for whom the oath was taken later....

‏البر والصلة والآداب‏ : Hadeeth No:6384 This hadith has been reported on the authority of Anas with a different chain of transmitters....