The seal of the Prophet ﷺ was made of iron with silver wound around it. Sometimes it would be on his hand. Muqayqib was in charge of the Prophet's ﷺ seal.
حدثنا ابن المثنى وزياد بن يحيى والحسن بن علي قالوا حدثنا سهل بن حماد أبو عتاب حدثنا أبو مكين نوح بن ربيعة
كان خاتم النبي ﷺ من حديد ملوي عليه فضة قال فربما كان في يده قال وكان المعيقيب على خاتم النبي ﷺ
No references are available.
Sourcing, [its chain is hasan]. Nasai narrated it in Kitab al-Zinah, Bab labs khatim hadid mulawwa alayhi bi-fiddah, no. 5208, from the hadith of Sahl bin Hammad, through this route.