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)

English translation · Sunan Abu Dawud