Shama-il Tirmidhi Chapter 012, Hadith Number 008 (097).

It is related from Hazrat Anas radiyallahu anhu that the Prophet of Allah sallallahu alaihe wasallam wore a ring on the right hand, and some people have also related from Hazrat Anas radiyallahu anhu that the Prophet of Allah sallallahu alaihe wasallam wore a ring on the left hand.

Commentary.
The findings of Imam Tirmizi are that both these hadith are incorrect, which would mean that Hazrat Anas radiyallahu anhu could not ascertain the correct hand. The Muhadditheen take great precautions and study each part of a hadith in detail to determine which portion is correct, and which is incorrect in that hadith, although in other ahaadith this has been ascertained to be correct consequently, today, the subject of hadith has been carefully preserved and has spread in the world with 'noor'. Many ahaadith prove that the Prophet of Allah sallallahu alaihe wasallam wore the ring on both the right or the left hand. The ahaadith where he wore it on the right hand have been mentioned in this chapter. The ahaadith regarding his wearing it on the left hand are stated in the Abu Daud and Muslim etc. Imam Nawawi informs that both type of ahaadith are correct. It is a practice of the Muhadditheen, that although if three ahaadith are correct (on a subject) and due to some special rule it does not comply with the standard, they will mention their views on this special rule. Therefore, Imam Tirmizi has made a comment here.