The Lord said to Moses:
You yourself are to speak to the Israelites: "You shall keep my sabbaths, for this is a sign between me and you throughout your generations, given in order that you may know that I, the Lord, sanctify you.
You shall keep the sabbath, because it is holy for you; everyone who profanes it shall be put to death; whoever does any work on it shall be cut off from among the people.
Six days shall work be done, but the seventh day is a sabbath of solemn rest, holy to the Lord; whoever does any work on the sabbath day shall be put to death.
Therefore the Israelites shall keep the sabbath, observing the sabbath throughout their generations, as a perpetual covenant.
It is a sign forever between me and the people of Israel that in six days the Lord made heaven and earth, and on the seventh day he rested, and was refreshed."
When God finished speaking with Moses on Mount Sinai, he gave him the two tablets of the covenant, tablets of stone, written with the finger of God.
References for Exodus 31:18