I do make them know the statutes of God, and his laws - These words are so very particular that they leave little room for doubt that the law had been given. Such words would scarcely have been used had not the statutes and laws been then in existence. And this is one of the proofs that the transaction mentioned here stands out of its due chronological order; See Clarke's note on Exodus 18:5.

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The Development of the Earlier Old Testament Laws
[Sidenote: First the principle, and then the detailed laws] If the canon of the New Testament had remained open as long as did that of the Old, there is little doubt that it also would have contained many laws, legal precedents, and ecclesiastical histories. From the writings of the Church Fathers and the records of the Catholic Church it is possible to conjecture what these in general would have been. The early history of Christianity illustrates the universal fact that the broad principles are …
Charles Foster Kent—The Origin & Permanent Value of the Old Testament