Exodus 31:15-19

For six days work may be done, but on the seventh day there is a Sabbath of complete rest, holy to the Lord; whoever does any work on the Sabbath day must be put to death.

So the sons of Israel shall keep the Sabbath, to celebrate the Sabbath throughout their generations as a permanent covenant.’

It is a sign between Me and the sons of Israel forever; for in six days the Lord made heaven and earth, but on the seventh day He ceased from labor, and was refreshed.”

When He had finished speaking with him on Mount Sinai, He gave Moses the two tablets of the testimony, tablets of stone, written by the finger of God.