Language:
كتاب فرض الخمس
57  
One-fifth of Booty to the Cause of Allah (Khumus)
(5)
Chapter: The armour of the Prophet saws
(5)
باب مَا ذُكِرَ مِنْ دِرْعِ النَّبِيِّ صلى الله عليه وسلم وَعَصَاهُ وَسَيْفِهِ وَقَدَحِهِ وَخَاتَمِهِ
Sahih al-Bukhari 3106

Narrated Anas:

That when Abu Bakr became the Caliph, he sent him to Bahrain and wrote this letter for him, and stamped it with the Ring of the Prophet. Three lines were engraved on the Ring, (the word) 'Muhammad' was in a line, 'Apostle' was in another line, and 'Allah' in a third.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ الأَنْصَارِيُّ، قَالَ حَدَّثَنِي أَبِي، عَنْ ثُمَامَةَ، عَنْ أَنَسٍ، أَنَّ أَبَا بَكْرٍ ـ رضى الله عنه ـ لَمَّا اسْتُخْلِفَ بَعَثَهُ إِلَى الْبَحْرَيْنِ، وَكَتَبَ لَهُ هَذَا الْكِتَابَ وَخَتَمَهُ، وَكَانَ نَقْشُ الْخَاتَمِ ثَلاَثَةَ أَسْطُرٍ مُحَمَّدٌ سَطْرٌ، وَرَسُولُ سَطْرٌ، وَاللَّهِ سَطْرٌ‏.‏
Reference : Sahih al-Bukhari 3106
In-book reference : Book 57, Hadith 15
USC-MSA web (English) reference : Vol. 4, Book 53, Hadith 338
  (deprecated numbering scheme)