(55) باب هَلْ يُجْعَلُ نَقْشُ الْخَاتَمِ ثَلاَثَةَ أَسْطُرٍ
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.
Reference: Sahih al-Bukhari 5878
In-book reference: Book 77, Hadith 95
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 72, Hadith 767