'According to the Law' in the Bible

Now, O king! wilt thou establish the interdict, and sign the writing, that it may not be changed - according to the law of the Medes and Persians, which may not be abolished,

Then drew they near, and began to speak before the king, concerning the royal interdict, Didst thou not sign, an interdict, that, any man who should petition of any God or man, for thirty days, save of thee, O king, should be cast into the den of lions? The king answered and said, Certain is the thing - according to the law of the Medes and Persians, which may not be abolished.