Musnad Ahmad 27:1256

Yahya ibn Hammad narrated to us, Abu 'Awanah narrated to us, from Abu Bishr, from Nafi', from Ibn 'Umar, who said: "The Prophet, peace and blessings of Allah be upon him, had a ring of gold. He used to place its stone toward the inside of his palm. Then he cast it away one day, so his companions cast away their rings. Then he took a ring of silver, and he used to seal with it, but he would not wear it."