וְעָשִׂ֤יתָ בַדִּים֙ לַמִּזְבֵּ֔חַ בַּדֵּ֖י עֲצֵ֣י שִׁטִּ֑ים וְצִפִּיתָ֥ אֹתָ֖ם נְחֹֽשֶׁת׃
I zrobisz drążki do ofiarnicy, - drążki z drzewa akacyowego; a obłożysz je miedzią.
Mekhilta DeRabbi Yishmael
Similarly, (Exodus 20:22) "And im ("when") an altar of stones you make unto Me." It is mandatory. You say it is mandatory, but perhaps it is optional? (Devarim 27:6) "Of whole stones shall you build the altar of your G–d" indicates that it is mandatory and not optional.