The Literal Commandment
"These words, which I am commanding you today, shall be on your heart. You shall teach them diligently to your sons and shall talk of them when you sit in your house and when you walk by the way and when you lie down and when you rise up. You shall bind them as a sign on your hand and they shall be as frontals on your forehead. (Deuteronomy 6:6-8)
The literal commandment is that we should bind G-d's words upon our hand.