It is narrated from Hazrat Anas (may Allah be pleased with him) that when the Prophet (peace and blessings be upon him) intended to write a letter to the Romans, the noble companions (may Allah be pleased with them) submitted that those people only read sealed letters. So the Prophet (peace and blessings be upon him) had a silver ring made. Its whiteness is still before my eyes. The inscription on it was: "Muhammad Rasul Allah (peace and blessings be upon him)".