Exodus 20:24-25

24You are to make for Me an altar of earth, and sacrifice on it your burnt offerings and peace offerings, your sheep and goats and cattle. In every place where I cause My name to be remembered, I will come to you and bless you.

25Now if you make an altar of stones for Me, you must not build it with stones shaped by tools; for if you use a chisel on it, you will defile it.