Sahih al-Bukhari 5878
صحيح البخاري#5878Sahihحَدَّثَنِي 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.
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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 77:95