The Qur'an is the Speech of Allah, If the Prophet forged anything against Allah, then Allah would punish him, meaning, `if Muhammad forged something against Us(Allah), as they(disbelievers) claim, and added or removed anything from the Message, or said anything from himself while attributing it to Us(Allah), then We(Allah) would surely be swift in punishing him. And of course, Muhammad did not do any of this (as the disbelievers claimed.