Abu Dawud Book 29, Hadith Number 4212.
Chapter : Signetring of iron.
Narated By Iyas b. al-Harith al-Mu’aiqib quoting his grandmother : The signet-ring of the Prophet (PBUH) 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 (PBUH).