Sunan an-Nasa'i, Book 48, Hadith 237 (In-Book) ⓘ
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).'"
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 .
also referenced under: Hadith 5276 (Reference) & Volume 6, Book 48, Hadith 5278 (DarusSalam)