Exodus 20:24-25

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.