Musnad Ahmad 30:1179

Abu Amir narrated to us, Abd al-Aziz ibn al-Muttalib narrated to us, from Abdullah ibn al-Hasan, from Abd al-Rahman al-A'raj, from Abu Hurairah, that the Prophet, peace and blessings of Allah be upon him, said: "Whoever has his wealth sought [from him] without right, and he is killed, then he is a martyr."