Ash-Shama'il Al-Muhammadiyah, Book 12, Hadith 90 (In-Book) ⓘ

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.

حَدَّثَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، قَالَ‏:‏ حَدَّثَنَا مُعَاذُ بْنُ هِشَامٍ، قَالَ‏:‏ حَدَّثَنِي أَبِي، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ‏:‏ لَمَّا أَرَادَ رَسُولُ اللهِ صلى الله عليه وسلم أَنْ يَكْتُبَ إِلَى الْعَجَمِ قِيلَ لَهُ‏:‏ إِنَّ الْعَجَمَ لا يَقْبَلُونَ إِلا كِتَابًا عَلَيْهِ خَاتَمٌ، فَاصْطَنَعَ خَاتَمًا، فَكَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي كَفِّهِ‏.‏