The Prophet ﷺ said, "When two Muslims raise weapons against each other, they both stand at the brink of Hellfire. If one of them kills the other, they will both enter it together."
عن النبي ﷺ قال: إذا المسلمان حمل أحدهما على أخيه السلاح، فهما في جرف جهنم، فإن قتل أحدهما صاحبه دخلاها جميعا.
Muslim narrated it in the chapter on tribulations (2888:16) via Muhammad bin Jafar, who said, Shuba narrated to us, from Mansur, from Rubai bin Khirash, from Abu Bakra, and he related it.Similar in meaning is what was reported from Abu Musa, who said, the Messenger of Allah ﷺ said, "When two Muslims meet with their swords, both the killer and the killed are in the Fire." They said, "O Messenger of Allah, this is understandable for the killer, but what about the one killed?" He said, "He intended to kill his companion."This was narrated by Nasai (4119), Ibn Majah (4:394), and Ahmad (19676), all of them via Hasan, from Abu Musa Ashari, and he related it. The wording given is that of Ibn Majah. Hasan here is Hasan Basri, and he did not hear directly from Abu Musa Ashari.Also similar in meaning is what was reported from Anas bin Malik, from the Prophet ﷺ, who said, "There are no two Muslims who meet with their swords except that both the killer and the killed are in the Fire." This was narrated by Ibn Majah (3963) and Aqeeli in Duafa (4/223), both of them via Mubarak bin Suhaym, from Abdul-Aziz bin Suhayb, from Anas, and he related it. Mubarak bin Suhaym is matruk (a discarded narrator).