One who fights so that the word of Allah will be supreme is fighting in the cause of Allah Sahih Muslim 1904 c
Arabic hadith text
وَحَدَّثَنَاهُ إِسْحَاقُ بْنُ إِبْرَاهِيمَ، أَخْبَرَنَا عِيسَى بْنُ يُونُسَ، حَدَّثَنَا الأَعْمَشُ، عَنْ
شَقِيقٍ، عَنْ أَبِي مُوسَى، قَالَ أَتَيْنَا رَسُولَ اللَّهِ صلى الله عليه وسلم فَقُلْنَا يَا رَسُولَ اللَّهِ
الرَّجُلُ يُقَاتِلُ مِنَّا شَجَاعَةً . فَذَكَرَ مِثْلَهُ
Translation
It has been narrated (through yet another chain of transmitters) on the same authority, i. e. Abu Musa, who said
We, came to the Messenger of Allah (ﷺ) and said: Messenger of Allah, one of us hho fights to display his valour... (followed by the same words as we have in the previous tradition).