How do We Know about the Mitzvah of Mezuzah?

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.