Hadith 2888 a - The Book of Tribulations and Portents of the Last Hour - Sahih Muslim - صحيح مسلم - Collection of Actions, Sayings and Teachings of Prophet Muhammad (صلى الله عليه و سلم) - Kutub as-Sittah

54. The Book of Tribulations and Portents of the Last Hour - كتاب الفتن وأشراط الساعة

4) Chapter: If Two Muslims Confront One Another With Their Swords

Ahnaf b. Qais reported:

I set out with the intention of helping this person (Hadrat 'Ali) that Abu Bakra met me. He said: Ahnaf, where do you intend to go? I said: I intend to help the cousin of Allah's Messenger (ﷺ), viz. 'Ali. Thereupon he said to me: Ahnaf, go back, for I heard Allah's Messenger (ﷺ) as saying: When two Muslims confront one another with swords (in hands) both the slayer and the slain would be in Fire. He (Ahnaf) said: I said, or it was said: Allah's Messenger, it may be the case of one who kills. but what about the slain (why he would be put in Hell-Fire)? Thereupon he said: He also intended to kill his companion.

Reference: 2888 a
English reference: Book 41, Hadith 6898
In-book reference: Book 54, Hadith 18
Grade: Sahih
Read this hadith from Darussalam Collection

4) باب إِذَا تَوَاجَهَ الْمُسْلِمَانِ بِسَيْفَيْهِمَا

حَدَّثَنِي أَبُو كَامِلٍ، فُضَيْلُ بْنُ حُسَيْنٍ الْجَحْدَرِيُّ حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ أَيُّوبَ، وَيُونُسَ عَنِ الْحَسَنِ، عَنِ الأَحْنَفِ بْنِ قَيْسٍ، قَالَ خَرَجْتُ وَأَنَا أُرِيدُ، هَذَا الرَّجُلَ فَلَقِيَنِي أَبُو بَكْرَةَ فَقَالَ أَيْنَ تُرِيدُ يَا أَحْنَفُ قَالَ قُلْتُ أُرِيدُ نَصْرَ ابْنِ عَمِّ رَسُولِ اللَّهِ صلى الله عليه وسلم - يَعْنِي عَلِيًّا - قَالَ فَقَالَ لِي يَا أَحْنَفُ ارْجِعْ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ إِذَا تَوَاجَهَ الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ ‏"‏ ‏.‏ قَالَ فَقُلْتُ أَوْ قِيلَ يَا رَسُولَ اللَّهِ هَذَا الْقَاتِلُ فَمَا بَالُ الْمَقْتُولِ قَالَ ‏"‏ إِنَّهُ قَدْ أَرَادَ قَتْلَ صَاحِبِهِ ‏"‏ ‏.‏