Musannaf Ibn Abi Shayba 16:868

Abu Bakr narrated to us, he said: Ibn 'Uyaynah narrated to us, from Ayyub ibn Musa, from Nafi', from Ibn 'Umar, who said: The Prophet, peace and blessings of Allah be upon him, took a ring made of silver, then he had engraved upon it "Muhammad is the Messenger of Allah." Then he said: "Let no one engrave upon the engraving of this ring of mine."