In Deuteronomy 18:20, God declares: “The prophet who speaks a word presumptuously in My name which I have not commanded him to speak . . . that prophet shall die.” So, if someone claims to be a prophet, but delivers a revelation that doesn’t really come from God, he’s a false prophet.
In the History of at-Tabari, Volume 6, page 111, Muhammad confesses: “I have fabricated things against God and have imputed to Him words which He has not spoken.” So, Muhammad, who claimed to be a prophet, admitted that he delivered a revelation that didn’t really come from God.
If both of these claims are true, Muhammad must be a false prophet.