Sayyiduna Anas (may Allah be pleased with him) said that when Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) sent him to Bahrain, he wrote a letter: 'Bismillahir-Rahmanir-Rahim' (In the name of Allah, the Most Gracious, the Most Merciful). "This is the zakah that Allah Almighty has made obligatory upon the people, which Allah Almighty commanded His Messenger (peace and blessings be upon him). Whoever asks the Muslims for it is to be given it, but if anyone demands more than that, it is not to be given to him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20411