Daniel 6:8

6:8 Now let the king issue a written interdict so that it cannot be altered, according to the law of the Medes and Persians, which cannot be changed.

Daniel 6:12

6:12 So they approached the king and said to him, “Did you not issue an edict to the effect that for the next thirty days anyone who prays to any god or human other than to you, O king, would be thrown into a den of lions?” The king replied, “That is correct, according to the law of the Medes and Persians, which cannot be changed.”

Daniel 6:15

6:15 Then those men came by collusion to the king and said to him, “Recall, O king, that it is a law of the Medes and Persians that no edict or decree that the king issues can be changed.”