Exodus 20:24-25

24“Build for me an altar made of earth, and offer your sacrifices to me—your burnt offerings and peace offerings, your sheep and goats, and your cattle. Build my altar wherever I cause my name to be remembered, and I will come to you and bless you.

25If you use stones to build my altar, use only natural, uncut stones. Do not shape the stones with a tool, for that would make the altar unfit for holy use.