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. (ES)
English reference (ES): Book 11, Hadith 86 (ES)
Arabic reference (ES): Book 12, Hadith 91 (ES)