It was narrated from Anas bin Malik that the Prophet ﷺ said:
“There are no two Muslims who confront one another with their swords, but both the killer and the slain will be in Hell.”

حَدَّثَنَا سُوَيْدُ بْنُ سَعِيدٍ، حَدَّثَنَا مُبَارَكُ بْنُ سُحَيْمٍ، عَنْ عَبْدِ الْعَزِيزِ بْنِ صُهَيْبٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، عَنِ النَّبِيِّ ﷺ قَالَ " مَا مِنْ مُسْلِمَيْنِ الْتَقَيَا بِأَسْيَافِهِمَا إِلاَّ كَانَ الْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ " .