Deuteronomy 27:6

Thou shalt build the altar of the Lord thy God of whole stones: and thou shalt offer burnt offerings thereon unto the Lord thy God:

Cross-reference

Exodus 20:25 contains the same prohibition against using hewn stones for the altar, which Deuteronomy echoes here.

Joshua 8:31 records the fulfillment of this command, as Joshua builds the altar of uncut stones and offers burnt offerings on Mount Ebal.