Sunan An-Nasa'i

It was narrated that Anas bin Malik said:""The Messenger of Allah [SAW] put on a ring, and he had some words inscribed on it. He said: 'We have put on a ring and have had some words engraved on it; no one of you should copy this inscription.'"" Then Anas said: ""It is as if I can see its whiteness on his hand