Deuteronomy 27:4-7

27:4 So when you cross the Jordan you must erect on Mount Ebal these stones about which I am commanding you today, and you must cover them with plaster. 27:5 Then you must build an altar there to the Lord your God, an altar of stones – do not use an iron tool on them. 27:6 You must build the altar of the Lord your God with whole stones and offer burnt offerings on it to the Lord your God. 27:7 Also you must offer fellowship offerings and eat them there, rejoicing before the Lord your God.