Shama-il Tirmidhi Chapter 012, Hadith Number 007 (096).

Imam Muhammad Baaqir relates that Hazrat Hasan radiyallhu anhu and Hazrat Husain radiyallahu anhu, both wore their rings on their left hands.

Commentary.
This hadith is contrary to the ones mentioned at the beginning of the chapter by Imam Tirmizi. It is stated there that the ring was worn on the right hand. An explanation can be given that the intention here is to indicate that after mentioning all the ahaadith, this narration that is contrary to the others may be of a weak chain. According to this humble servant, the mentioning of the right hand shows that this is the preferred method. This narration (under discussion) indicates that this too is permissible. Some of the learned have explained that in translations, according to the practice of the Muhadditheen, the word, 'am fee yasaarihi' has been removed i.e. the Prophet of Allah sallallahu alaihe wasallam used to wear the ring on the right hand or the left. According to this explanation no narration will be different from a translation.