We know from the written Torah about the commandment to affix Mezuzah (the commandment d’Oraita).
We read about it every day while reciting the Shema Israel: “And write them (these signs) on the doorposts of your house, and on your gate” (Dvarim 6: 5-9 ). The word mezuzah literally translates as a doorpost or gatepost.
On the doorposts it is commanded to write these signs. What are these signs? These are the Torah verses (Dvarim 6: 5-9) in which this law is written.