(15) باب إِذَا بُويِعَ لِخَلِيفَتَيْنِ
It 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.
Reference: Sahih Muslim 1853
In-book reference: Book 33, Hadith 96
USC-MSA web (English) reference(deprecated numbering scheme): Book 20, Hadith 4568