Abdullah bin Buraydah narrates from his father (Sayyiduna Buraydah, may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whomever we appoint as an official, we will provide him with sustenance; whatever (expense) he takes beyond that, he is a betrayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 13020