“See that the Lord has given you the Shabbat; therefore, He gives you on the sixth day bread for two days. Remain each man where he is; no man shall leave his place on the seventh day (Exodus 16:29). “See that the Lord has given you the Shabbat.” It should have said “know.”
What is “see”? Rather, this is what the Holy One blessed be He said to them: ‘If the idolaters were to come and say to you: Why do you observe the Shabbat day on this day, say to them: See that the manna does not fall on Shabbat.’ What is “has given you”? To you it is given, and not to idolaters.
From here they said: If idolaters will come and observe Shabbat, not only do they not receive a reward, [they also deserve to be put to death], as it is stated: “And day and night will not cease [yishbotu]” (Genesis 8:22).24This verse is interpreted to mean: They shall not observe a day and night of Shabbat. Likewise it says: “Between Me and the children of Israel [it is a sign forever]” (Exodus 31:17).
This is analogous to a king who is sitting and the queen is sitting opposite him; one who passes between them is liable.