There are many examples of God's laws being followed without it being recorded when God first gave them such as with
Genesis 39:9 where Joseph knew that it was a sin to commit adultery, so the fact that the first recorded instance of a command against adultery wasn't until Sinai does not give justification for assuming that there was no law against adultery prior to that, and the same is true for the Sabbath.
The Ten Commandments, which Sabbath is part of, were not given until Exodus 16.
Not Abraham, not Isaac, not Jacob, none of the patriarchs kept Sabbath (Fathers prior to Egypt did not keep the Sabbath:
Deut 5:2-3 "The Lord our God made a covenant with us at Horeb. The Lord did not make this covenant with our fathers, but with us, with all those of us alive here today."). The first time the Sabbath is mentioned in some significant way is in the 16th chapter of Exodus, when God feeds the people manna from heaven as they wander in the wilderness.
And the manna comes every day except the Sabbath day, and the day before they get enough for that day, so that they don’t have to work on that day. And that gives them a little preview of what’s coming, because in the 20th chapter you have the Ten Commandments, and in the Ten Commandments, prescriptions are given that do set down laws for the Sabbath day. This is the first time any such laws have been given by God.
The Sabbath was not instituted for man in Genesis. It was instituted officially in Exodus, in the law of Moses. A further understanding of that comes from Exodus chapter 31. The Lord speaks to Moses in verse 12, and He says to him, “As for you, speak to the sons of Israel saying, ‘You shall surely observe My Sabbaths; for this is a sign between Me and you throughout your generations, that you may know that I am the Lord who sanctifies you.
“‘Therefore you are to observe the Sabbath, for it is holy to you. Everyone who profanes it shall surely be put to death; for whoever does any work on it, that person shall be cut off from among his people. For six days work may be done but on the seventh day there is a Sabbath of complete rest, holy to the Lord; whoever does any work on the Sabbath day shall surely be put to death. So the sons of Israel shall observe the Sabbath, to celebrate the Sabbath throughout their generations as a perpetual covenant.
“‘It is a sign between Me and the sons of Israel forever;’” - why? - “‘for in six days the Lord made heaven and earth, but on the seventh day He ceased, and was refreshed,’” - or
rested. Here we find that Sabbath is a sign; it is a sign. That is to say, it points to something else. It is a symbol, connected to the Mosaic covenant.
When God made a covenant with Noah, He promised Noah that He would never destroy the world again, and God identified a sign. What was the sign of the Noahic covenant? Rainbow. When God made a covenant with Abraham, He made that covenant with Abraham and He designated a sign, and the sign of the Abrahamic covenant, participation among the covenant people Israel, was the sign of circumcision. And here you have in the Mosaic covenant another sign, and the sign this time is the Sabbath.
It was only a sign. Observing it with a duplicitous heart gained nothing. In fact,
Isaiah 1:13 says, “Bring your worthless offerings no longer, Incense is an abomination to Me. New moon and Sabbath.” The prophet Hosea pronounces a similar judgment on their hypocritical Sabbaths: “I will put an end to all her gaiety, her feasts, her new moons, her Sabbaths.” It didn’t mean anything to observe it outwardly without a heart of love and devotion to God.
The Sabbath was a reminder of creation. The Sabbath was to remind the people of Israel that they had forfeited paradise; that man had forfeited paradise. The law said to them, “Obey this law and you will be blessed.” God said that repeatedly: “Obey this law and you will be blessed,” to show them that righteous behaviour would restore a taste of Eden’s paradise. Righteous behaviour would also point to a future, a future kingdom when paradise would be regained.
So, the Sabbath, every Sabbath that went by, when they rested, they were reminded of a perfect creation, a paradise of God dominated by righteousness, which had been forfeited by sin and could only be regained again by righteousness. God then institutes the seventh-day system - not for everybody in the world; in fact, specifically, it says, for Israel. Verse 17: “A sign between Me and the sons of Israel forever.” Every seventh day was a reminder that they were living in a fallen world. Every seventh day was a reminder that they had lost paradise.
The Sabbath was the sign to Israel of the Old Covenant. Because we are now under the New Covenant, we are no longer required to keep the sign of the Old Covenant. The New Testament nowhere commands Christians to observe the Sabbath.
Paul warned the Gentiles about many different sins in his epistles, but never about breaking the Sabbath.
The early church Fathers, from Ignitions to Augustine, taught that the Old Testament Sabbath had been abolished and that the first day of the week (Sunday) was the day when Christians should meet for worship.