Exodus 20:24-26

24 You shall make an altar of earth for me, and shall sacrifice on it your burnt offerings and your peace-offerings, your sheep and your oxen. In every place where I record my name I will come to you and I will bless you.  25 If you make me an altar of stone, you shall not build it of hewn stones; for if you lift up your tool on it, you have polluted it.  26 Neither shall you go up by steps to my altar, that your nakedness may not be exposed to it.'