Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that when the Noble Prophet (peace and blessings be upon him) intended to write a letter to the Romans, the people said to him that they only read a letter which has a seal on it. So the Noble Prophet (peace and blessings be upon him) had a silver ring made. It is as if I am still seeing the whiteness of that ring in the hand of the Noble Prophet (peace and blessings be upon him). The words "Muhammad Rasul Allah" were engraved on it.