It is related from Hazrat Anas radiallahu anhu that the inscription engraved on the ring of Rasulullah sallallahu alayhe wasallam was 'Muhammad' on one line, 'Rasul' on a line, and 'Allah' on a line. (PT)
English reference (PT): Book 11, Hadith 86 (PT)
Arabic reference (PT): Book 12, Hadith 91 (PT)