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.
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 " .
Muslim
The Book on Government • Hadith 96