Daniel 6:14-15

Then the king, when he heard these words, was sore displeased with himself, and set his heart on Daniel to deliver him: and he laboured till the going down of the sun to deliver him.

Then these men assembled unto the king, and said unto the king, Know, O king, that the law of the Medes and Persians is, That no decree nor statute which the king establisheth may be changed.