It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Whoever's wealth is being taken unjustly and he is killed defending it is a martyr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16778