Shama-il Tirmidhi

Anas radiallahu anhu relates: "When Rasulullah sallallahu alayhe wasallam intended to write letters to the kings of 'Ajam (non arabs), inviting them to Islam, the people said that they did not accept letters without a seal (stamp) on them. Rasulullah sallallahu alayhe wasallam therefore had a ring made, the whiteness of which is still before my eyes."

Commentary.
The last sentence shows that he remembers this incident well. By whiteness the silver in the ring is referred to.