The Prophet ﷺ said, "When two Muslims raise weapons against each other, they are both standing at the brink of Hellfire. If one kills the other, they will both enter it together."
و حدثنا أبو بكر بن أبي شيبة حدثنا غندر عن شعبة ح و حدثنا محمد بن المثنى وابن بشار قالا حدثنا محمد بن جعفر حدثنا شعبة عن منصور عن ربعي بن حراش
عن النبي ﷺ قال إذا المسلمان حمل أحدهما على أخيه السلاح فهما على جرف جهنم فإذا قتل أحدهما صاحبه دخلاها جميعا
Muslim included the hadith of Ghundar, from Shubah, from Mansur, from Rubai, from Abu Bakra, from the Prophet, peace and blessings be upon him and his family, "When two Muslims meet with their swords, both are on the brink of Hell, and if they kill each other, they both enter it." Bukhari mentioned it as a suspended narration (muallaq), saying that Ghundar and Shababa both narrated it, and he said that Thawri did not narrate it from Mansur as a statement attributed to the Prophet (marfu).