If the Prophet forged anything against Allah, then Allah would punish Him

Allah says,

وَلَوْ تَقَوَّلَ عَلَيْنَا

فَسَبِّحْ بِاسْمِ رَ‌بِّكَ الْعَظِيمِ 'So, pronounce the purity of the name of your magnificent Lord. [ 69:52] '. It was shown in the preceding verses that Allah's Messenger ﷺ never speak from his own whims and fancies. He only conveys pure revelation revealed to him by Allah. The Divine Word is a reminder and an admonition for the God-conscious people. Allah knows that despite knowing the absolutely incontestable truth many people will continue to reject it. Consequently, it will be a source of eternal remorse, disappointment and punishment in the Hereafter for the disbelievers. In the end, Allah says:

وَإِنَّهُ لَحَقُّ الْيَقِينِ 'And it is surely an absolute truth. [ 69:51] '. In other words, the Qur'an is undeniably the Truth of Certainty. There is no room for any doubt or suspicion in it.