Deuteronomy 27:3 (KJV)

And Moses with the elders of Israel commanded the people, saying, Keep all the commandments which I command you this day.

And it shall be on the day when ye shall pass over Jordan unto the land which the Lord thy God giveth thee, that thou shalt set thee up great stones, and plaister them with plaister:

And thou shalt write upon them all the words of this law, when thou art passed over, that thou mayest go in unto the land which the Lord thy God giveth thee, a land that floweth with milk and honey; as the Lord God of thy fathers hath promised thee.

Therefore it shall be when ye be gone over Jordan, that ye shall set up these stones, which I command you this day, in mount Ebal, and thou shalt plaister them with plaister.

And there shalt thou build an altar unto the Lord thy God, an altar of stones: thou shalt not lift up any iron tool upon them.