Amr bin 'Uthman عمرو بن عثمان بن عفان · The Book of Adornment

"The Prophet wore a ring of gold, then he discarded it and wore a ring of silver on which were engraved (the words) 'Muhammad Rasul Allah.' Then he said: 'No one should copy this inscription of mine.' And he wore the stone (Fass) toward his palm."