Sayyiduna Anas (may Allah be pleased with him) said that when Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) became caliph, he sent him to Bahrain as an official, wrote this letter for him and sealed it with the ring of the Prophet (peace and blessings be upon him). The engraving of his (peace and blessings be upon him) ring was in three lines: in one line was written Muhammad (peace and blessings be upon him), in the second Messenger, and in the third Allah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7247