Negative commandment · Lo Ta’aseh #79 of 613
Not to build the altar with hewn stones
שלא לבנות אבני מזבח גזית
The verse · Shemot 20:22
וְאִם־מִזְבַּח אֲבָנִים תַּעֲשֶׂה־לִּי לֹא־תִבְנֶה אֶתְהֶן גָּזִית כִּי חַרְבְּךָ הֵנַפְתָּ עָלֶיהָ וַתְּחַלְלֶהָ
“And if thou wilt make me an altar of stone, thou shalt not build it of hewn stone: for if thou lift up thy tool upon it, thou hast defiled it.”
The Rambam’s words
Not to build the altar with hewn stones, as [Exodus 20:22] states: "Do not make them hewn."
שלא לבנות אבני מזבח גזית, שנאמר "לֹא תִבְנֶה אֶתְהֶן גָּזִית" (שמות כ, כא).
Why · Sefer HaChinuch #40
To not build with hewn stones
“It is from the roots of this commandment that we should fix into our souls from the day that we make [the altar], that forgiveness of iniquity, blessing and peace will come [through] it afterwards.”
Who & when
“And [it] is practiced at the time of the [Temple] by males and females.”
Where in the Mishneh Torah
Hilchot The Chosen Temple הלכות בית הבחירה