Chapter 36: Signet-Rings (Kitab Al-Khatam)
Iyas b. al-Harith b. al-Mu' aiqib quoting his grandfather said and his grandfather from his mother' s side was Abu Dhubab: The signet-ring of the Prophet (ﷺ) was of iron polished with silver. Sometimes it remained in my possession. Al-Mu' ayqib was in charge of the signet-ring of the Prophet (ﷺ).
Sunan Abi Dawud 4224