The true prophets are not permitted to declare their own words or personal opinion as the word from the mouth of the Lord. Divine authority must never be ascribed to their own word or else the message will be corrupted by human opinion and desires, which is the case with the false prophets and teachers.

“For I have not spoken on My own authority; but the Father who sent Me gave Me a command, what I should say and what I should speak. 50 And I know that His command is everlasting life. Therefore, whatever I speak, just as the Father has told Me, so I speak.”
John 12:49-50 (AMP)

21 for no prophecy was ever made by an act of human will, but men moved by the Holy Spirit spoke from God.
2 Peter 1:21