'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 may not be revoked.

Then they came near, and spoke before the king concerning the king's decree: Hast thou not signed a decree, that every man that shall ask anything of any god or man within thirty days, except 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 may not be revoked.