Sunan an-Nasa'i, Book 37, Hadith 151 (In-Book) ⓘ
It was narrated that Abu Bakrah said: "The Messenger of Allah [SAW] said: 'If a Muslim points a weapon at his fellow Muslim, then they are on the brink of Hell, and if he kills him, then they will both fall into it.'"
أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، قَالَ حَدَّثَنَا أَبُو دَاوُدَ، عَنْ شُعْبَةَ، قَالَ أَخْبَرَنِي مَنْصُورٌ، قَالَ سَمِعْتُ رِبْعِيًّا، يُحَدِّثُ عَنْ أَبِي بَكْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا أَشَارَ الْمُسْلِمُ عَلَى أَخِيهِ الْمُسْلِمِ بِالسِّلاَحِ فَهُمَا عَلَى جُرُفِ جَهَنَّمَ فَإِذَا قَتَلَهُ خَرَّا جَمِيعًا فِيهَا " .
also referenced under: Hadith 4116 (Reference) & Volume 5, Book 37, Hadith 4121 (DarusSalam)