Sahih al-Bukhari 3106
Sahih al-Bukhari · Book 57: One-fifth of Booty to the Cause of Allah (Khumus) · What has been said regarding the armour of the Prophet ﷺ, his staff, sword, cup and ring, and what the caliphs after him used of these things which had not been distributed, and also (what is said) about his hair, shoes and utensils which were considered as blessed things by his companions and others after his death
Narrated Anas:
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.
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.