Exodus 16:23-30 And he said to them, This is that which Yahweh has said, Tomorrow is the rest of the holy sabbath to Yahweh. Bake what you will bake today, and boil what you will boil. And that which remains over, lay up for you to be kept until the morning. (24) And they laid it up until the morning, as Moses said. And it did not stink, neither was there any worm in it. (25) And Moses said, Eat that today. For today is a sabbath to Yahweh. Today you shall not find it in the field. (26) Six days you shall gather it, but on the seventh day, the sabbath, in it there shall be none. (27) And it happened some of the people went out on the seventh day in order to gather. And they did not find any. (28) And Yahweh said to Moses, How long do you refuse to keep My commandments and My Laws? (29) See, because Yahweh has given you the sabbath, therefore He gives you the bread of two days on the sixth day. Each one stay in his place. Let not any one go out of his place on the seventh day. (30) So the people rested on the seventh day.
Before Israel arrived at Sinai, before God had spoken a single word from the mountain top or carved a single letter on the stone tablets, he said, “How long do you refuse to keep my mitzvot and torah?” God expected Israel to obey his laws, specifically the law of the Sabbath in this case, before he had a covenant with them and before he had given them the whole law.
Before that, God commended Abraham for heeding his call, keeping his charge, his commandments (mitzvot), his statutes (khukot), and his laws (torot). Most people interpret that to mean the Noahide laws, but Noah certainly had more laws than those. How else would he know what animals were clean and unclean? How did Abel know what kind of animal to sacrifice, and how should Cain have known that his sacrifice would be unacceptable?
God’s laws are eternal and not tied to any particular covenant. When you enter your neighbor’s house, he expects you to observe the rules of his house: Don’t play football in the living room, don’t put your feet on the furniture, don’t open the refrigerator without an invitation, etc. This doesn’t mean that he invented those rules the moment you walked in the door. They were always the rules of his house because they are a part of his character. He doesn’t have anything against your shoes in particular; he just doesn’t like it when people put their shoes on his sofa. God’s laws are the same. They are a reflection of his unchanging character. One can make a case (a very weak case, in my opinion) that god invented the laws concerning tabernacle rituals and the Levitical priesthood arbitrarily or only for the specific nature of the Israelites, but one cannot make the same case regarding sabbath, animals that are acceptable for food and sacrifice, and behavior toward your neighbors. God’s standards in those matters all clearly existed before Sinai and will continue to exist so long as heaven and earth remain.