Exodus 20:24-26

24An altar of earth shalt thou make unto me, and shalt sacrifice on it thy burnt-offerings, and thy peace-offerings, thy sheep and thine oxen: in all places where I shall make my name to be remembered, I will come unto thee, and bless thee. 25And if thou make me an altar of stone, thou shalt not build it of hewn stone; for if thou lift up thy sharp tool upon it, thou hast profaned it. 26Neither shalt thou go up by steps unto mine altar, that thy nakedness be not discovered thereon.