Sahih al-Bukhari 65

Muhammad ibn Muqatil Abu al-Hasan narrated to us. He said: Abdullah informed us. He said: Shu'ba informed us, from Qatada, from Anas ibn Malik, who said: The Prophet (peace and blessings be upon him) wrote a letter—or intended to write one. He was told: ‘They do not read a letter unless it is sealed.’ So he had a silver ring made, engraved: ‘Muhammad, Messenger of Allah.’ It is as though I can see its whiteness in his hand. I said to Qatada: ‘Who said, “It was engraved: Muhammad, Messenger of Allah”?’ He said: ‘Anas.’