"But the prophet who shall purposely speak a word in My name which I have not commanded him to speak"Deuteronomy 18:20.

It is forbidden to utter a false prophecy in G‑d's name—i.e. to state that G‑d has said something that He has not. It is also forbidden for someone to say a (true) prophecy, stating that this is something that G‑d has revealed to him, when in fact G‑d spoke this prophecy to another prophet.