Exodus 31:12-16

12Then Adonai spoke to Moses saying,

13“Speak now to Bnei-Yisrael saying, ‘Surely you must keep My Shabbatot, for it is a sign between Me and you throughout your generations, so you may know that I am Adonai who sanctifies you.

14Therefore you are to keep the Shabbat, because it is holy for you. Everyone who profanes it will die, for whoever does any work during Shabbat, that soul will be cut off from the midst of his people.

15Work is to be done for six days, but on the seventh day is a Shabbat of complete rest, holy to Adonai. Whoever does any work on the Shabbat will surely be put to death.

16So Bnei-Yisrael is to keep the Shabbat, to observe the Shabbat throughout their generations as a perpetual covenant.