Hadith 77:95
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.
حَدَّثَنِي مُحَمَّدُ بْنُ عَبْدِ اللَّهِ الأَنْصَارِيُّ، قَالَ حَدَّثَنِي أَبِي، عَنْ ثُمَامَةَ، عَنْ أَنَسٍ، أَنَّ أَبَا بَكْرٍ ـ رضى الله عنه ـ لَمَّا اسْتُخْلِفَ كَتَبَ لَهُ، وَكَانَ نَقْشُ الْخَاتَمِ ثَلاَثَةَ أَسْطُرٍ. مُحَمَّدٌ سَطْرٌ، وَرَسُولُ سَطْرٌ، وَاللَّهِ سَطْرٌ
Original Reference: Sahih al-Bukhari 5878