Hazrat Anas bin Malik (may Allah be pleased with him) narrates that when the Messenger of Allah (peace and blessings be upon him) wrote a letter (to invite a king to Islam) or intended to write one, it was said to him that they do not read a letter without a seal (i.e., they do not consider an unsealed letter as authentic). So the Prophet (peace and blessings be upon him) had a silver ring made, on which "Muhammad Rasul Allah" (Muhammad, the Messenger of Allah) was engraved. It is as if I (even today) can see the whiteness of it shining on the Prophet’s (peace and blessings be upon him) hand.