Deuteronomy 27:5

And thou shalt build there an altar to the Lord thy God, an altar of stones; thou shalt not lift up iron upon it.

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.

There you shall build an altar to Yahweh your God, an altar of stones. You shall not use any iron tool on them.