Exodus 31:15-17

15Six days shall work be done; but on the seventh day is the sabbath of rest, holy to Jehovah: whoever doeth work on the sabbath day shall certainly be put to death.

16And the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations — [it is] an everlasting covenant.

17It shall be a sign between me and the children of Israel for ever; for [in] six days Jehovah made the heavens and the earth, and on the seventh day he rested, and was refreshed.