And there you shall build an altar to the LORD your God, an altar of stones; you shall not use an iron tool on them.
Deuteronomy 27:5
Surrounding context
27:4 Therefore it shall be, when you have crossed over the Jordan, that on Mount Ebal you shall set up these stones, which I command you today, and you shall whitewash them with lime.
27:5 And there you shall build an altar to the LORD your God, an altar of stones; you shall not use an iron tool on them.
27:6 You shall build with whole stones the altar of the LORD your God, and offer burnt offerings on it to the LORD your God.