Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "Neither the Messenger of Allah (peace and blessings be upon him), nor Abu Bakr (may Allah be pleased with him), nor Umar (may Allah be pleased with him) used to wear rings, nor did they seal any letter," until Ziyad bin Abi Sufyan wrote to Umar bin al-Khattab (may Allah be pleased with him): You write to us about matters that carry no seals. So at that he had a seal made and began sealing with it.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13068