'According to the Law' in the Bible

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

Then they came near, and spake before the king concerning the king's decree; Hast thou not signed a decree, that every man that shall ask a petition of any God or man within thirty days, save of thee, O king, shall be cast into the den of lions? The king answered and said, The thing is true, according to the law of the Medes and Persians, which altereth not.

But the men who had gone as a group to the king told him, "Remember, your majesty, that according to the laws of the Medes and Persians, any decree or edict that the king establishes cannot be repealed."