Musannaf Ibn Abi Shayba 18:1325

Abu Bakr narrated to us, he said: Marwan narrated to us, from Yazid ibn Sinan, from Maymun ibn Mihran, from Ibn Umar, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever is killed defending his wealth, he is a martyr."