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.
However God is clearly saying in the same passage that we ought to ask each other what the Lord is saying. It is the attitude expressed in the response that matters.
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 it will speak for itself.