ENOfficial
It was narrated that Abu Bakrah said: "If two Muslim men bear weapons against each other, then they are both on the brink of Hell. And if one of them kills the other, they will both be in Hell."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا أَحْمَدُ بْنُ سُلَيْمَانَ، قَالَ حَدَّثَنَا يَعْلَى، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ مَنْصُورٍ، عَنْ رِبْعِيٍّ، عَنْ أَبِي بَكْرَةَ، قَالَ إِذَا حَمَلَ الرَّجُلاَنِ الْمُسْلِمَانِ السِّلاَحَ أَحَدُهُمَا عَلَى الآخَرِ فَهُمَا عَلَى جُرُفِ جَهَنَّمَ فَإِذَا قَتَلَ أَحَدُهُمَا الآخَرَ فَهُمَا فِي النَّارِ .
sunnah.com