This accusation is against our faith and it is never found in any of our books. On the contrary, we believe that any doubt in the Prophet Muhammad's prophet hood makes the person a non Muslim.
It is nothing but a false allegation like hundreds of false allegations fabricated against Shia Islam by their enemies who don't have any evidence against Shia Islam, so they fabricate false allegations to mislead lay persons who don't research nor verify. Same situation is with the allegations of enemies of Islam and Quran against all Muslims to prevent non Muslims from understanding Islam.
These allegations against us have no value for those who follow evidence but they prove that our opponents are helpless and unable to counter our concrete evidence, so they try mislead people by creating and spreading false allegations.