Sahih Muslim 4799
صحيح مسلم#4799SahihIt has been narrated on the authority of Aba Sa'id al-Khudri that the Messenger of Allah (ﷺ) said:When oath of allegiance has been taken for two caliphs, kill the one for whom the oath was taken later
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 33:96