What did God at first reveal unto man as the rule of his obedience?

The rule of obedience revealed to Adam in the estate of innocence, and to all mankind in him, besides a special command not to eat of the fruit of the tree of the knowledge of good and evil, was the moral law.[1] 

1. Gen. 1:26-27

   Then God said, “Let us make man in our image, after our likeness. And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth.”

   So God created man in his own image,
   in the image of God he created him;
   male and female he created them.

   Genesis 2:17

   …but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.

   Rom. 2:14-15

   For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. 15 They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them…

   Rom. 10:5

   For Moses writes about the righteousness that is based on the law, that the person who does the commandments shall live by them.

Question 92, Westminster Larger Catechism