Musnad Ahmad 2:69

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah narrated to us, from 'Ata' ibn Dinar, from Abu Yazid al-Khawlani, who said: I heard Fudalah ibn 'Ubayd saying: I heard 'Umar ibn al-Khattab saying: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "The martyrs are four: A man who is a believer, of good faith, who met the enemy and was truthful to Allah, so he was killed; that is the one whom people look up to like this—and he raised his head until the cap of the Messenger of Allah (peace and blessings of Allah be upon him) fell, or the cap of 'Umar. The second: A man who is a believer who met the enemy, and it was as if his back was being struck by the thorns of the acacia tree, then a stray arrow came to him and killed him; that is in the second rank. The third: A man who is a believer who mixed a righteous deed with another that was evil, who met the enemy and was truthful to Allah, the Almighty and Majestic, until he was killed; that is in the third rank. The fourth: A man who is a believer who transgressed greatly against his own soul, who met the enemy and was truthful to Allah until he was killed; that is in the fourth rank."