'According to the Law' in the Bible

Now, O king, establish the decree, and sign the writing, that it not be changed, according to the law of the Medes and Persians, which cannot be changed."

Then they came near, and spoke before the king concerning the king's decree. "Haven't you signed a decree, that every man who shall make petition to any god or man within thirty days, except to you, O king, shall be cast into the den of lions?" The king answered, "The thing is true, according to the law of the Medes and Persians, which cannot be changed."