“But the prophet, that shall speak a word presumptuously in My name, which I have not commanded him to speak…” (Devarim 18:20)
It is a negative commandment not to prophesy falsely. This means that no man may say that things were said to him as a prophesy from Hashem when as a matter of fact Hashem had not said them. This prohibition includes even if a person knows of a prophesy received by a prophet and this man says falsely that they were said to him. About all of this it says “But the prophet, that shall speak a word presumptuously in My name, which I have not commanded him to speak…”, and Chazal explain that when the verse says “that shall speak a word presumptuously in My name,” this refers to one who says a prophesy which he had not heard and “which I have not commanded him to speak” refers to one who says over a prophesy which was not said to him but to someone else.
Amongst the roots of this commandment are that if this was not prohibited this could cause a great destruction and much evil in our holy complete religion, for the essence of the real truth without any impurities can only reach people through the prophets, and the Torah commanded us to believe and trust them and to follow their correct advice and their complete knowledge. it follows that when wicked people arise and say things that Hashem had not commanded them they will cause people to slander prophesy, which is a great fundamental amongst us, the holy nation, and because of them people will come to doubt even the true prophets. If a person says something that someone else has prophesied this also causes a great destruction, for this person when saying that he is a prophet and has been commanded to say or do certain things, and they see that his words are coming true like those of the true prophets they will assume that he is also a holy man of G-d; a messenger of Hashem, and they will believe in him and trust him and use him as a role model and example to follow all of his actions in everything that we do, and in actual fact maybe, since his actual merit and behavior are not enough to warrant him being a messenger about this thing, we may not rely on him in all that he does and says, and the general public will err by following his advice.
This commandment applies in all place and at all times, both to men and to women. One who transgresses this and prophesies falsely, whether he says things in Hashem’s name which Hashem had not said, and whether he says things which Hashem had said to someone else and no to him, deserves the death penalty.