Anas (may Allah be pleased with him) relates:
"When Messenger of Allah 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. Messenger of Allah 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 ﷺ أَنْ يَكْتُبَ إِلَى الْعَجَمِ قِيلَ لَهُ: إِنَّ الْعَجَمَ لا يَقْبَلُونَ إِلا كِتَابًا عَلَيْهِ خَاتَمٌ، فَاصْطَنَعَ خَاتَمًا، فَكَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي كَفِّهِ.