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 gave him this document: "Bismillahir-Rahmanir-Rahim (In the name of Allah, the Most Gracious, the Most Merciful). These are the obligatory rates of zakah which Allah has made obligatory upon the Muslims and which Allah commanded His Messenger (peace and blessings be upon him) about. So whoever is asked for zakah in accordance with it should pay it, and whoever is asked for more than this amount (the due rate) should not pay it." And he narrated the whole hadith to the end, and said: "On silver, a quarter of a tenth is due; when it is one hundred and ninety, there is no zakah on it, except what its owner wishes to give."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7519