Musnad Abu Ya'la al-Mawsili 243:33

Narrated to us Muhriz ibn 'Awn, narrated to us 'Ali ibn Mushir, from al-A'mash, from Shaqiq, from Abu Musa al-Ash'ari, who said: A man came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, a man fights out of bravery, and he fights out of zeal, and he fights for show—which of those is in the way of Allah?" The Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever fights so that the word of Allah may be supreme, then he is in the way of Allah, the Mighty and Majestic."