Mishkat al-Masabih 18 The Offices of Commander and Qadi كتاب الإمارة والقضاء 16

وَعَنْ أَبِي سَعِيدٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِذَا بُويِعَ لِخَلِيفَتَيْنِ فاقتُلوا الآخِرَ منهُما» . رَوَاهُ مُسلم

Abu Sa’id al-Khudri reported God’s Messenger as saying, “When allegiance is sworn to two caliphs kill the second of them.* Muslim transmitted it. * Some interpret ‘kill’ here as meaning to fight with. The tradition is directed against the possibility of two Caliphs holding power at the same time, even though the areas might be quite distinct.