4 And who, you ask, shall help me, save He who slumbers not, nor sleeps? Hear what follows: “The Lord Himself is your keeper”. It is not therefore man, that slumbers and sleeps, but the Lord, that keeps you. How does He keep you? “The Lord is your defence upon the hand of your right hand.”...It seems to me to have a hidden sense: otherwise he would have simply said, without qualification, “The Lord will keep you,” without adding, “on your right hand.” For how? Does God keep our right hand, and not our left? Did He not create the whole of us? Did not He who made our right hand, make our left hand also? Finally, if it pleased Him to speak of the right hand alone, why said He, “on the hand of your right hand,” and not at once “upon your right hand”? Why should He say this, unless He were keeping somewhat here hidden for us to arrive at by knocking? For He would either say, “The Lord shall keep you,” and add no more; or if He would add the right hand, “The Lord shall keep you upon your right hand;” or at least, as He added “hand,” He would say, “The Lord shall keep you upon your hand, even your right hand,” not “upon the hand of your right hand.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)