Sunan an-Nasa'i, Volume 6, Book 48, Hadith 5202 (DarusSalam) ⓘ
It was narrated from Anas that: The Prophet's ring was made of silver and its stone (Fass) was made of silver too.
أَخْبَرَنَا أَبُو بَكْرِ بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا أُمَيَّةُ بْنُ بِسْطَامٍ، قَالَ حَدَّثَنَا مُعْتَمِرٌ، قَالَ سَمِعْتُ حُمَيْدًا، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ خَاتَمُهُ مِنْ وَرِقٍ فَصُّهُ مِنْهُ .
also referenced under: Hadith 5199 (Reference) & Book 48, Hadith 160 (In-Book)