Sahih Bukhari Volume 1, Book 2, Hadith 31

حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ الْمُبَارَكِ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، حَدَّثَنَا أَيُّوبُ، وَيُونُسُ، عَنِ الْحَسَنِ، عَنِ الأَحْنَفِ بْنِ قَيْسٍ، قَالَ ذَهَبْتُ لأَنْصُرَ هَذَا الرَّجُلَ، فَلَقِيَنِي أَبُو بَكْرَةَ فَقَالَ أَيْنَ تُرِيدُ قُلْتُ أَنْصُرُ هَذَا الرَّجُلَ‏.‏ قَالَ ارْجِعْ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ إِذَا الْتَقَى الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ ‏"‏‏.‏ فَقُلْتُ يَا رَسُولَ اللَّهِ هَذَا الْقَاتِلُ فَمَا بَالُ الْمَقْتُولِ قَالَ ‏"‏ إِنَّهُ كَانَ حَرِيصًا عَلَى قَتْلِ صَاحِبِهِ ‏"

Narrated By Al-Ahnaf bin Qais : While I was going to help this man ('Ali Ibn Abi Talib), Abu Bakra met me and asked, "Where are you going?" I replied, "I am going to help that person." He said, "Go back for I have heard Allah's Apostle saying, 'When two Muslims fight (meet) each other with their swords, both the murderer as well as the murdered will go to the Hell-fire.' I said, 'O Allah's Apostle! It is all right for the murderer but what about the murdered one?' Allah's Apostle replied, "He surely had the intention to kill his companion."