Hadith: Whoever fights so that the word of Allah be supreme is indeed fighting in the cause of Allah.

عن أبي موسى الأشعري رضي الله عنه قال: «سُئِلَ رَسُولُ الله صلى الله عليه وسلم عَنْ الرَّجُلِ: يُقَاتِلُ شَجَاعَةً، وَيُقَاتِلُ حَمِيَّةً، وَيُقَاتِلُ رِيَاءً، أَيُّ ذَلِكَ فِي سَبِيلِ الله؟ فَقَالَ رَسُولُ الله صلى الله عليه وسلم : مَنْ قَاتَلَ لِتَكُونَ كَلِمَةُ الله هِيَ الْعُلْيَا، فَهُوَ فِي سَبِيلِ الله».
[صحيح] - [متفق عليه]

Abu Mūsa al-Ash‘ari (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) was asked about a man who fights out of valor, a man who fights out of chauvinism, and a man who fights out of ostentation, which one fights in the cause of Allah? He replied: "Whoever fights so that the word of Allah be supreme is indeed fighting in the cause of Allah."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

A man asked the Prophet (may Allah's peace and blessings be upon him) about a man who fights the enemies of Islam, but the motive to fight is to appear brave and bold in front of people; and about a man who fights fanatically for his people or his country; and a third one who fights to show off as one who fights in the cause of Allah and who deserves praise and reverence. So, who of these three is the real fighter in the cause of Allah? He (may Allah's peace and blessings be upon him) replied with the most concise phrase implying the most comprehensive meaning: "Whoever fights so that the word of Allah be supreme is indeed fighting in the cause of Allah." Other than this is not in the cause of Allah because he fights for another purpose. Deeds depend on intentions for their validity and invalidity, which is a general criteria for all deeds because what counts for deeds to be accepted or not is the intention, and the proofs of this meaning are numerous.