Hadith 1853
Chapter 15: When allegiance has been sworn to two caliphs - كتاب الإمارة
وَحَدَّثَنِي وَهْبُ بْنُ بَقِيَّةَ الْوَاسِطِيُّ، حَدَّثَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ، عَنِ الْجُرَيْرِيِّ، عَنْ أَبِي، نَضْرَةَ عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا بُويِعَ لِخَلِيفَتَيْنِ فَاقْتُلُوا الآخَرَ مِنْهُمَا " .
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 : Book 20, Hadith 4568