The inscription on the seal of the Messenger of Allah ﷺ was, "Muhammad, Messenger of Allah."
أخبرنا إسحق بن إبراهيم قال أنبأنا محمد بن بشر قال حدثنا عبيد الله عن نافع
كان نقش خاتم رسول الله ﷺ محمد رسول الله
No references are available.
Muslim narrated it in the Book of Clothing, chapter "The Prophet ﷺ Wearing a Silver Ring," etc, hadith no. 2091/54, from the hadith of Ubaydullah bin Umar, in a longer form.