It is narrated from Hazrat Anas (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) had a ring made of silver for himself and said: We have had a ring made and have engraved on it the inscription "Muhammad Rasul Allah" (Muhammad, the Messenger of Allah), therefore no one should engrave this inscription on his ring.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12941