25 “I have heard what the prophets say who prophesy lies in my name. They say, ‘I had a dream! I had a dream!’ 26 How long will this continue in the hearts of these lying prophets, who prophesy the delusions of their own minds? 27 They think the dreams they tell one another will make my people forget my name, just as their ancestors forgot my name through Baal worship. 28 Let the prophet who has a dream recount the dream, but let the one who has my word speak it faithfully. For what has straw to do with grain?” declares the Lord. 29 “Is not my word like fire,” declares the Lord, “and like a hammer that breaks a rock in pieces?
30 “Therefore,” declares the Lord, “I am against the prophets who steal from one another words supposedly from me. 31 Yes,” declares the Lord, “I am against the prophets who wag their own tongues and yet declare, ‘The Lord declares.’ 32 Indeed, I am against those who prophesy false dreams,” declares the Lord. “They tell them and lead my people astray with their reckless lies, yet I did not send or appoint them. They do not benefit these people in the least,” declares the Lord.
33 “When these people, or a prophet or a priest, ask you, ‘What is the message from the Lord?’ say to them, ‘What message? I will forsake you, declares the Lord.’ 34 If a prophet or a priest or anyone else claims, ‘This is a message from the Lord,’ I will punish them and their household. 35 This is what each of you keeps saying to your friends and other Israelites: ‘What is the Lord’s answer?’ or ‘What has the Lord spoken?’ 36 But you must not mention ‘a message from the Lord’ again, because each one’s word becomes their own message. So you distort the words of the living God, the Lord Almighty, our God. 37 This is what you keep saying to a prophet: ‘What is the Lord’s answer to you?’ or ‘What has the Lord spoken?’ 38 Although you claim, ‘This is a message from the Lord,’ this is what the Lord says: You used the words, ‘This is a message from the Lord,’ even though I told you that you must not claim, ‘This is a message from the Lord.’ 39 Therefore, I will surely forget you and cast you out of my presence along with the city I gave to you and your ancestors. 40 I will bring on you everlasting disgrace—everlasting shame that will not be forgotten.”
The Lord makes a point about prophesying in the name of the Lord in Jeremiah 23:34-40. He says that that phrase: “Thus says the Lord” and others like it have been misused so much that they now bring dishonor to His name. People have used this kind of phrase to give authority to their own ideas and in so doing have turned the words of God on their head.
In Jeremiah 31:34 God says no one needs to teach those who know the Lord. You don’t need to say “Thus says the Lord” or anything like that to a listening ear. If it is the word of the Lord He can speak for Himself.