Musnad Ahmad 5:29

Abu Yusuf al-Mu'addib Ya'qub, our neighbor, narrated to us, saying: Ibrahim ibn Sa'd narrated to us, from 'Abd al-'Aziz ibn al-Muttalib, from 'Abd al-Rahman ibn al-Harith, from Zayd ibn 'Ali ibn al-Husayn, from his father, from his grandfather, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever is killed defending his wealth, he is a martyr."