Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 15
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 15, with English translation by Maimonides (Rambam).
לִקְבֹּעַ מְזוּזָה, שֶׁנֶּאֱמָר: "וּכְתַבְתָּם עַל מְזוּזֹת בֵּיתֶךָ וּבִשְׁעָרֶיךָ" (דברים ו ט; יא, כ).
To affix a mezuzah, as [Deuteronomy 6:9] states: "And you shall write them on the doorposts of your home."