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.  

أَنَّ أَبَا بَكْرٍ ؓ لَمَّا اسْتُخْلِفَ كَتَبَ لَهُ وَكَانَ نَقْشُ الْخَاتَمِ ثَلاَثَةَ أَسْطُرٍ مُحَمَّدٌ سَطْرٌ وَرَسُولُ سَطْرٌ وَاللَّهِ سَطْرٌ  

The ring of the Prophet ﷺ was in his hand, and after him, in Abu Bakr's hand, and then in ʿUmar's hand after Abu Bakr. When ʿUthman was the Caliph, once he was sitting at the well of Aris. He removed the ring from his hand and while he was trifling with it, dropped into the well. We kept on going to the well with ʿUthman for three days looking for the ring, and finally the well was drained, but the ring was not found.  

كَانَ خَاتَمُ النَّبِيِّ ﷺ فِي يَدِهِ وَفِي يَدِ أَبِي بَكْرٍ بَعْدَهُ وَفِي يَدِ عُمَرَ بَعْدَ أَبِي بَكْرٍ فَلَمَّا كَانَ عُثْمَانُ جَلَسَ عَلَى بِئْرِ أَرِيسَ قَالَ فَأَخْرَجَ الْخَاتَمَ فَجَعَلَ يَعْبَثُ بِهِ فَسَقَطَ قَالَ فَاخْتَلَفْنَا ثَلاَثَةَ أَيَّامٍ مَعَ عُثْمَانَ فَنَنْزَحُ الْبِئْرَ فَلَمْ نَجِدْهُ