(Exodus 16:28) "And the L–rd said to Moses: How long will you refuse to keep, etc.": R. Yehoshua says: The Holy One Blessed be He said to Moses: Moses, say to Israel: I took you out of Egypt and I split the sea for you, and I brought down the manna for you, and I brought up the well, and I swept in the quail for you, and I fought the war with Amalek for you, and I wrought miracles and mighty acts for you — "How long will you refuse to keep My mitzvoth and My laws?"
Lest you say that I have imposed too many mitzvoth upon you, this Sabbath (alone) did I charge you with at Marah to keep it, and you have not kept it! Lest you say: What reward is received by the keeper of the Sabbath? It is written (Isaiah 56:2) "Happy is the man who does this, and the son of man who holds fast to it — who keeps Sabbath not to profane it — He keeps his hand from doing all evil." We are hereby apprised that one who keeps the Sabbath is kept far from transgression.