Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 79
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 79, with English translation by Maimonides (Rambam).
שלא לבנות אבני מזבח גזית, שנאמר "לֹא תִבְנֶה אֶתְהֶן גָּזִית" (שמות כ, כא).
Not to build the altar with hewn stones, as [Exodus 20:22] states: "Do not make them hewn."