Sayyiduna Mu'ayqib (may Allah be pleased with him) said: The seal-ring of the Messenger of Allah (peace and blessings be upon him) was of iron overlaid with silver, and sometimes it would be in my hand. And Sayyiduna Mu'ayqib (may Allah be pleased with him) was in charge of the seal of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17956