Ahnaf b. Qais said:
I came out with the intention of (participating in) fighting. Abu Bakrah met me and said: Go back, for I heard the Messenger of Allah ﷺ say: When two Muslims face each other with their swords, the killer and the slain will go to Hell. He asked: Messenger of Allah, this is the killer (so naturally he should go to Hell), but what is the matter with the slain ? He replied: He intended to kill his companion.

حَدَّثَنَا أَبُو كَامِلٍ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ أَيُّوبَ، وَيُونُسَ، عَنِ الْحَسَنِ، عَنِ الأَحْنَفِ بْنِ قَيْسٍ، قَالَ خَرَجْتُ وَأَنَا أُرِيدُ، - يَعْنِي فِي الْقِتَالِ - فَلَقِيَنِي أَبُو بَكْرَةَ فَقَالَ ارْجِعْ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ " إِذَا تَوَاجَهَ الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ " . قَالَ يَا رَسُولَ اللَّهِ هَذَا الْقَاتِلُ فَمَا بَالُ الْمَقْتُولِ قَالَ " إِنَّهُ أَرَادَ قَتْلَ صَاحِبِهِ " .