Abu Dawud
Signet Rings
Book Number: 029 / Hadith Number: 4215
Narated By Abdullah ibn Umar : The Prophet (pbuh) used to wear the signet-ring on his left hand, and put its stone next the palm of his hand.
Abu Dawud said: Ibn Ishaq and Usamah b. Zaid transmitted from Nafi': "on his right hand."