Sahih Muslim Book - 33, Hadith - 4922

وحَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ ، أَخْبَرَنَا جَرِيرٌ ، عَنْ مَنْصُورٍ ، عَنْ أَبِي وَائِلٍ ، عَنْ أَبِي مُوسَى الأَشْعَرِيِّ ، أَنَّ رَجُلاً سَأَلَ رَسُولَ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ، عَنِ الْقِتَالِ فِي سَبِيلِ اللهِ عَزَّ وَجَلَّ ، فَقَالَ الرَّجُلُ : يُقَاتِلُ غَضَبًا ، وَيُقَاتِلُ حَمِيَّةً ، قَالَ : فَرَفَعَ رَأْسَهُ إِلَيْهِ ، وَمَا رَفَعَ رَأْسَهُ إِلَيْهِ إِلاَّ أَنَّهُ كَانَ قَائِمًا ، فَقَالَ : مَنْ قَاتَلَ لِتَكُونَ كَلِمَةُ اللهِ هِيَ الْعُلْيَا ، فَهُوَ فِي سَبِيلِ اللَّهِ.

It was narrated from Abu Musa Al-Ash'ari that a man asked the Messenger of Allah (s.a.w) about fighting in the cause of Allah, glorified and exalted is He. He said: "A man may fight out of anger, or for the sake of his tribe." He raised his head to look at him - and he only raised his head because the man was standing - and said: "The one who fights so that the word of Allah will be supreme is (fighting) in the cause of Allah."