'According to the Law' in the Bible

Now, O king, establish the ban and sign the writing, so that it may 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 ban, Have you not signed a ban that every man who shall ask a petition of any god or man within thirty days, except of you, O king, shall be thrown into the lion's den? The king answered and said, The thing is true, according to the law of the Medes and Persians, which cannot be changed.