Abu Musa (May Allah be pleased with him) reported: The Prophet (PBUH) said, "The honest Muslim trustee who carries out duties assigned to him (in another narration he said, "Who gives''), and he gives that in full, with his heart overflowing with cheerfulness, to whom he is ordered, he is one of the two givers of charity".
[Al-Bukhari and Muslim].
This Hadith also mentions a very important principle to the effect that everyone must wholeheartedly perform the duty assigned to him by the state. If he is appointed custodian of the Bait-ul-Mal (public exchequer), he must fulfill his duty with utmost honesty. If he is not envious of anyone, nor harms anybody, nor behaves niggardly then he will be entitled to the same reward which will be due to the person who entrusted him that duty. Thus this Hadith enjoins every Muslim to perform his official duties wholeheartedly and to the best of his ability.