Narrated Abu Musa Al-Ash`ari:A bedouin asked the Prophet, "A man may fight for the sake of booty, and another may fight so that he may be mentioned by the people, and a third may fight to show his position (i.e. bravery); which of these regarded as fighting in Allah's Cause?" The Prophet (ﷺ) said, "He who fights so that Allah's Word (i.e. Islam) should be superior, fights for Allah's Cause."
حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرٍو، قَالَ سَمِعْتُ أَبَا وَائِلٍ، قَالَ حَدَّثَنَا أَبُو مُوسَى الأَشْعَرِيُّ ـ رضى الله عنه ـ قَالَ قَالَ أَعْرَابِيٌّ لِلنَّبِيِّ صلى الله عليه وسلم الرَّجُلُ يُقَاتِلُ لِلْمَغْنَمِ، وَالرَّجُلُ يُقَاتِلُ لِيُذْكَرَ، وَيُقَاتِلُ لِيُرَى مَكَانُهُ، مَنْ فِي سَبِيلِ اللَّهِ فَقَالَ " مَنْ قَاتَلَ لِتَكُونَ كَلِمَةُ اللَّهِ هِيَ الْعُلْيَا فَهْوَ فِي سَبِيلِ اللَّهِ ".
Bukhari
One-fifth of Booty to the Cause of Allah (Khumus) • Hadith 35