It is narrated from Sayyiduna Khalid bin Sa'id (may Allah be pleased with him) that he came to the Prophet (peace and blessings be upon him) wearing a ring on his hand, and the Prophet (peace and blessings be upon him) said: "O Khalid, what is this ring?" He said: It is a ring I have had made. He (peace and blessings be upon him) said: "Throw it to me." He said: So I threw it to him, and it was a ring of coloured iron overlaid with silver. The Prophet (peace and blessings be upon him) asked: "What is engraved on it?" I said: 'Muhammad Rasulullah' (Muhammad is the Messenger of Allah). So the Prophet (peace and blessings be upon him) took the ring and wore it, and this was the ring that remained on his hand (thereafter).
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4103