Musnad Ahmad 28:340

Waki' narrated to us, from Sufyan, from Abdullah ibn al-Hasan, from his maternal uncle Ibrahim ibn Muhammad ibn Talhah, from Abdullah ibn Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever has his wealth sought [from him] without right, and he is killed defending it, then he is a martyr."