Sahih al-Bukhari, Volume 7, Book 72, Hadith 767 (USC-MSA) ⓘ

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.‏

Anas added: The ring of the Prophet was in his hand, and after him, in Abu Bakr's hand, and then in `Umar's hand after Abu Bakr. When `Uthman 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 `Uthman for three days looking for the ring, and finally the well was drained, but the ring was not found.

وَزَادَنِي أَحْمَدُ حَدَّثَنَا الأَنْصَارِيُّ، قَالَ حَدَّثَنِي أَبِي، عَنْ ثُمَامَةَ، عَنْ أَنَسٍ، قَالَ كَانَ خَاتَمُ النَّبِيِّ صلى الله عليه وسلم فِي يَدِهِ، وَفِي يَدِ أَبِي بَكْرٍ بَعْدَهُ، وَفِي يَدِ عُمَرَ بَعْدَ أَبِي بَكْرٍ، فَلَمَّا كَانَ عُثْمَانُ جَلَسَ عَلَى بِئْرِ أَرِيسَ ـ قَالَ ـ فَأَخْرَجَ الْخَاتَمَ، فَجَعَلَ يَعْبَثُ بِهِ فَسَقَطَ قَالَ فَاخْتَلَفْنَا ثَلاَثَةَ أَيَّامٍ مَعَ عُثْمَانَ فَنَنْزَحُ الْبِئْرَ فَلَمْ نَجِدْهُ‏.‏