Sahih Bukhari Volume 4, Book 53, Hadith Number 338.

Narated By Anas : That when Abu Bakr became the Caliph, he sent him to Bahrain and wrote this letter for him, and stamped it with the Ring of the Prophet. Three lines were engraved on the Ring, (the word) ‘Muhammad’ was in a line, ‘Apostle’ was in another line, and ‘Allah’ in a third.