Exodus 19:12

19:12 You must set boundaries for the people all around, saying, ‘Take heed to yourselves not to go up on the mountain nor touch its edge. Whoever touches the mountain will surely be put to death!

Exodus 31:15

31:15 Six days work may be done, but on the seventh day is a Sabbath of complete rest, holy to the Lord; anyone who does work on the Sabbath day must surely be put to death.

Exodus 35:2

35:2 In six days work may be done, but on the seventh day there must be a holy day for you, a Sabbath of complete rest to the Lord. Anyone who does work on it will be put to death.