Sahih Muslim 1853
Sahih Muslim · Book 33: The Book on Government · When allegiance has been sworn to two caliphs
It has been narrated on the authority of Aba Sa'id al-Khudri that the Messenger of Allah (ﷺ) said:
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 " .
When oath of allegiance has been taken for two caliphs, kill the one for whom the oath was taken later.