Shama-il Tirmidhi Chapter 011, Hadith Number 005 (086).

It is related from Hazrat Anas radiallahu anhu that the inscription engraved on the ring of Rasulullah sallallahu alayhe wasallam was 'Muhammad Rasullulah, of in the first line was engraved, in the second line 'Rasul, and in the third line 'Allah'.

Commentary.
Some 'ulama have written that 'Muhammad Rasulullah' was engraved in such a manner, that the word 'Allah' was engraved on the top. This stamp was round, and was read from the bottom. The muhaqqiqeen (research scholars) write that this is not proven from any hadith, but from the apparent words we find it was written thus: Muhammad... Rasul... Allah.