The Right and the Left in the Koran
The Right (hand or companions) and the Left (hand or companions) refer to the two groups of people on Resurrection Day indicating good persons on the right (Yamin) and bad ones on the left (Shemal) .
In the following Koranic verses the both groups are mentioned:
Anyone who is given his book in his right (Yamin) hand will be called to account with on easy reckoning Return joyfully to his family while anyone who is given his book behind his back /will appeal to be blotted out and will roast in the Blaze. (84:7-12)
“Behind his back” refers to the left hand, indicating sinner’s punishment.
On that day you will (all) be arraigned, no secret of yours will remain hidden. Anyone who is given his book in his right hand will say: “Here, read my book! I always thought I would face my reckoning”! He will be in pleasant living in a lofty garden whose clusters [of fruit] will hang within easy reach. “Eat and drink to your heart’s content because of what you sent on ahead in bygone days.” However anyone who is given his book in his left hand will say: “it’s bad for me, if only my book had not been given me, I would not have known my reckoning! If it had only been the sentence [once for all]! My money has not helped me out; my authority has been wiped out on me.” (69:18-29)
Sources :
Recognition of Resurrection (Allameh Tehrani)
The Noble Quran (English translation by T.B. Irving)