The 15th mitzvah is that we are commanded in the actions1 involving the mezuzah.

The source of this commandment is G‑d's statement (exalted be He),2 "And you shall write them upon the doorposts of your house and upon your gates."

This identical command is repeated in the Torah.3

All the details of this mitzvah have been explained in the third chapter of tractate Menachos.4