Isaiah 8:20 - To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.
And BTW, you have again taken one verse out of context:
Isaiah 8:19-22
19 And when they say to you, "Consult the mediums and the spiritists who whisper and mutter," should not a people consult their God? Should they consult the dead on behalf of the living? 20 To the law and to the testimony! If they do not speak according to this word, it is because they have no dawn. 21 And they will pass through the land hard-pressed and famished, and it will turn out that when they are hungry, they will be enraged and curse their king and their God as they face upward. 22 Then they will look to the earth, and behold, distress and darkness, the gloom of anguish; and they will be driven away into darkness. NAS