Shama-il Tirmidhi Chapter 012, Hadith Number 006 (095).

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."

Commentary.
The reason for the Prophet of Allah sallallahu alaihe wasallam prohibiting the Sahabah radiyallahu anhum to make the same inscription on their rings is because the Sahabah radiyallahu anhum followed and imitated the Prophet of Allah sallallahu alaihe wasallam in every sphere of life, and by doing so it was feared that there would be a confusion with the original seal. Hazrat Mu'ayqib radiyallahu anhu was a Sahabi and the guardian (keeper) of the ring of the Prophet of Allah sallallahu alaihe wasallam. When the Prophet of Allah sallallahu alaihe wasallam did not wear the ring it was kept by Hazrat Mu'ayqib radiyallahu anhu. In the same manner it was kept by him during the reign of Hazrat Abubakr, Hazrat Umar and Hazrat Uthman radiyallah anhum. During the reign of Hazrat Uthman radiyallahu anhu, while sitting at the well of Arees, Hazrat Mu'ayqib radiyallahu anhu was giving the ring to Hazrat Uthman radiyallah anhu or was taking it from him and it accidentally fell into the well. This incident is mentioned in many ahaadith. As to whether it fell from the hands of Hazrat Uthman radiyallahu anhu or from the hands of Mu'ayqib radiyallahu anhu, the Ulama have adopted the view that it was the latter. Both ahaadith agree that it happened between them.