Hazrat Ibn Umar radiyallahu anhu reports, "The Prophet of Allah sallallahu alaihe wasallam had a ring made of silver. The top portion remained towards the inside. On it was engraved 'Muhammad the Rasoolullah'. The people were prohibited from making this inscription on their rings. This is the same ring which fell from the hands of Hazrat Mu'ayqib radiyallahu anhu into the well of Arees during the reign of Hazrat Uthman radiyallahu anhu.
English reference: Book 12, Hadith 95
Arabic reference: Book 13, Hadith 101