Hadith 2390
Sayyiduna Ali (may Allah be pleased with him) narrates that I said to Sayyiduna Abbas (may Allah be pleased with him) to request the Noble Prophet (peace and blessings be upon him) to appoint you as the collector of Zakat. (Upon his request) the Prophet (peace and blessings be upon him) said: "I will not appoint you over the filth of people's sins."