Exodus 31:14-15

31:14 So you must keep the Sabbath, for it is holy for you. Everyone who defiles it must surely be put to death; indeed, if anyone does any work on it, then that person will be cut off from among his people. 31:15 Six days work may be done, but on the seventh day is a Sabbath of complete rest, holy to the Lord; anyone who does work on the Sabbath day must surely be put to death.