Sayyiduna Anas bin Malik (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) intended to write to the Romans, it was said to him (peace and blessings be upon him): They do not read a letter without a seal. So he (peace and blessings be upon him) had a silver ring made, and its inscription was 'Muhammad Rasul Allah' (Muhammad is the Messenger of Allah). Sayyiduna Anas (may Allah be pleased with him) said: It is as if I am looking at its whiteness in the hand of the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20413