Exodus 31:15-18
15Six days may work be done; but in the seventh is the sabbath of rest, holy to the LORD: whosoever doeth any work in the sabbath day, he shall surely be put to death.
16Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant.
17It is a sign between me and the children of Israel for ever: for in six days the LORD made heaven and earth, and on the seventh day he rested, and was refreshed.
18And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, two tables of testimony, tables of stone, written with the finger of God.
(KJV)