(52) باب اتِّخَاذُ الْخَاتَمِ لِيُخْتَمَ بِهِ الشَّىْءُ، أَوْ لِيُكْتَبَ بِهِ إِلَى أَهْلِ الْكِتَابِ وَغَيْرِهِمْ
Narrated Anas bin Malik:
When the Prophet (ﷺ) intended to write to the Byzantines, it was said to him, "Those people do not read your letter unless it is stamped." So the Prophet (ﷺ) took a silver ring and got 'Muhammad, the Apostle of Allah' engraved on it .... as if I am now looking at its glitter in his hand.
Reference: Sahih al-Bukhari 5875
In-book reference: Book 77, Hadith 92
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 72, Hadith 764