Mukhtasar Muslim

Narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Noble Prophet (peace and blessings of Allah be upon him) wanted to write letters to (the king of Iran) Kisra, (the king of Rome) Caesar, and (the king of Abyssinia) Najashi. The people said, "These kings will not accept any letter unless it has a seal on it." So the Prophet (peace and blessings of Allah be upon him) had a ring made, the band of which was of silver, and "Muhammad Rasul Allah" was engraved on it.