Sayyidah Umm Salamah (may Allah be pleased with her) said: Once he (peace and blessings be upon him) was at home, and some Companions were sitting with him (peace and blessings be upon him) talking, when suddenly a man came and said: "O Messenger of Allah (peace and blessings be upon him), how much is the zakah on such-and-such amount of dates?" The Messenger of Allah (peace and blessings be upon him) told him: "Such-and-such." Then the man said: "So-and-so transgressed against me; he took such-and-such amount of zakah and took one sa' extra." The Messenger of Allah (peace and blessings be upon him) said: "How will it be when there are those over you who transgress even more than this?" The people became engrossed in this matter and the discussion grew, until a man asked: "O Messenger of Allah, if someone is far from you, among his camels, livestock and fields, and he keeps paying the zakah of his wealth, then he is wronged in his due, what should he do when he is far from you?" The Messenger of Allah (peace and blessings be upon him) said: "Whoever pays the zakah of his wealth willingly, seeking only the pleasure of Allah and the Hereafter, hides nothing of his wealth and establishes the prayer, then he is wronged, and he takes up his weapon, fights and is killed, he is a martyr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7532