Did the Sabbath start at Mount Sinai, or is it even older than that? This verse offers a fascinating clue.
The instructions in Exodus 16:23 about the Sabbath aren't a brand-new law being introduced, but rather a clarification and re-emphasis of a practice already known. Moses explains that 'Tomorrow is the rest of the holy Sabbath to the LORD.' This phrasing suggests that the concept of a holy day of rest was not entirely foreign to the Israelites, even before the formal Ten Commandments were given.
Commentators point out that the people's actions and their surprise at the double portion of manna on the sixth day imply a prior understanding of the Sabbath principle. If it were entirely new, how would Moses know to instruct them about gathering extra, or how would they even grasp the significance of the seventh day being different?
This points to the Sabbath being an ordinance established at creation, as mentioned in Genesis. While its formal observance and legal codification came with the Law at Sinai, the underlying principle of setting aside a day for rest and worship seems to have roots much deeper in human history.