Dan Daniel, chapter Book of Daniel, chapter 6
Then they came near, and spoke before the king concerning the kingʼs decree: Hast thou not signed a decree, that every man that shall make a petition of any g-d or man within shloshim yom, 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 cannot be changed.