Musnad Ahmad 788:254

Hassan ibn Musa narrated to us, he said: Zuhayr narrated to us, he said: Mansur ibn al-Mu'tamir narrated to us, from Abu Wa'il, he said: Abu Musa said: A man asked, or a man came to the Messenger of Allah, may Allah bless him and grant him peace, while the Messenger of Allah, may Allah bless him and grant him peace, was sitting with his head bowed. He said: "What is fighting in the way of Allah, the Almighty and Majestic? For one of us fights out of tribal zeal and out of anger, so does he have a reward?" He said: The Messenger of Allah, may Allah bless him and grant him peace, raised his head toward him—and had he not been standing, or had he been sitting (the doubt is from Zuhayr), he would not have raised his head toward him—and he said: "Whoever fights so that the word of Allah may be supreme, then he is in the way of Allah, the Almighty and Majestic."