Deuteronomy 27:5-8

5Moreover, you shall build there an altar to the Lord your God, an altar of stones; you 1shall not wield an iron tool on them. 6You shall build the altar of the Lord your God of 2uncut stones, and you shall offer on it burnt offerings to the Lord your God; 7and you shall sacrifice peace offerings and eat there, and 3rejoice before the Lord your God. 8You shall write on the 4stones all the words of this Law very clearly.”