Sahih Muslim | Book 20 | Chapter 15

Narrated by It has been narrated on the authority of Aba Sa'id al-Khudri that the Messenger of Allah (may peace be upon him)
It has been narrated on the authority of Aba Sa'id al-Khudri that the Messenger of Allah (may peace be upon him) said: When oath of allegiance has been taken for two caliphs, kill the one for whom the oath was taken later.