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 ‏.‏

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." Translation Not Available

Book Ref: Sunan An Nisai Book 37 Hadith 4117
Web Ref:  Sunan An Nisai Vol 5 Book 37 Hadith 4122