ENOfficial
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.
Source: sunnah.com
Other translations
Arabic
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 " .
sunnah.com