From Sahih Bukhari (Hadith 3126):
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 is 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
Reference: Bukhari 3126