The Book of Adornment
48/78. Chapter:
Description of the Ring of the Prophet [SAW] and its Inscription
Sunan an-Nasai
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا مُحَمَّدُ بْنُ بِشْرٍ، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ كَانَ نَقْشُ خَاتَمِ رَسُولِ اللَّهِ صلى الله عليه وسلم مُحَمَّدٌ رَسُولُ اللَّهِ .
It was narrated that Ibn 'Umar said: "The inscription on the ring of the Messenger of Allah [SAW] was: Muhammad Rasul Allah (Muhammad the Messenger of Allah).'"
Reference : Sunan an-Nasa'i 5276In-book reference : Book 48, Hadith 237English translation : Vol. 6, Book 48, Hadith 5278