This is from his work Economy of the Covenants Between God and Man, pg. 65 Vol. I:
For the law approves of nothing which it did not command, condemns nothing which it did not forbid. The law is Torah, the doctrine of right and wrong. What it teaches to be evil, that it forbids; what to be good, it commands. And therefore it is deservedly called the law of nature; not only because nature can make it known; but also because it is the rule of nature itself.