Musnad Ahmad 28:552

Ya'qub ibn Ibrahim narrated to us, 'Abd al-'Aziz—meaning Ibn al-Muttalib al-Makhzumi—narrated to us, from 'Abd al-'Aziz ibn 'Umar ibn 'Abd al-'Aziz, from 'Amr ibn Shu'ayb al-Sahmi, from his father, from his grandfather, from the Prophet, peace and blessings of Allah be upon him, that he said: "Whoever is killed defending his wealth is a martyr."