Narrated Anas: that when Abu Bakr became the Caliph, he wrote a letter to him (and stamped it with the Prophet's ring) and the engraving of the ring was in three lines: Muhammad in one line, 'Apostle' in another line, and 'Allah' in a third line.
Hadith ReferenceSahih Bukhari / The Book of Dress / Hadith: 5878
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة