Albert Barnes Commentary Jeremiah 23:31

"Behold, I am against the prophets, saith Jehovah, that use their tongues, and say, He saith." — Jeremiah 23:31 (ASV)

That use their tongues - literally, that take their tongues. Their second characteristic. They have no message from God, but they take their tongues, their only implement, and say, He says, using the solemn formula by which Yahweh affirms the truth of His words. Solemn asseverations seemed to give reality to their emptiness.