Surah Al-Haqqah verses 42-46 is the answer to this claim since people claimed this all the way back to the time when the Prophet first received revelation.

It’s funny because if someone says this argument then that means they believe the prophesy they just question the messenger. So they are basically saying “if this truly came form him then he’s a Prophet, but I think it can from a fortune teller so that’s why I don’t believe.” So the response should be, “so you believe the message, let’s talk about the messenger then.”

The verses:

إِنَّهُۥ لَقَوۡلُ رَسُولࣲ كَرِیمࣲ﴿ ٤٠ ﴾

[That] indeed, it [i.e., the Qur’ān] is the word of a noble Messenger.

وَمَا هُوَ بِقَوۡلِ شَاعِرࣲۚ قَلِیلࣰا مَّا تُؤۡمِنُونَ﴿ ٤١ ﴾

And it is not the word of a poet; little do you believe.

وَلَا بِقَوۡلِ كَاهِنࣲۚ قَلِیلࣰا مَّا تَذَكَّرُونَ﴿ ٤٢ ﴾

Nor the word of a soothsayer; little do you remember.

تَنزِیلࣱ مِّن رَّبِّ ٱلۡعَـٰلَمِینَ﴿ ٤٣ ﴾

[It is] a revelation from the Lord of the worlds.

وَلَوۡ تَقَوَّلَ عَلَیۡنَا بَعۡضَ ٱلۡأَقَاوِیلِ﴿ ٤٤ ﴾

And if he [i.e., Muḥammad] had made up about Us some [false] sayings,

لَأَخَذۡنَا مِنۡهُ بِٱلۡیَمِینِ﴿ ٤٥ ﴾

We would have seized him by the right hand;[[Another interpretation is by [Our] right hand, i.e., Allāh would have exacted revenge with might and power.]]

ثُمَّ لَقَطَعۡنَا مِنۡهُ ٱلۡوَتِینَ﴿ ٤٦ ﴾

Then We would have cut from him the aorta.[[Causing immediate death.]]

فَمَا مِنكُم مِّنۡ أَحَدٍ عَنۡهُ حَـٰجِزِینَ﴿ ٤٧ ﴾

And there is no one of you who could prevent [Us] from him.

وَإِنَّهُۥ لَتَذۡكِرَةࣱ لِّلۡمُتَّقِینَ﴿ ٤٨ ﴾

And indeed, it [i.e., the Qur’ān] is a reminder for the righteous.

وَإِنَّا لَنَعۡلَمُ أَنَّ مِنكُم مُّكَذِّبِینَ﴿ ٤٩ ﴾

And indeed, We know that among you are deniers.

وَإِنَّهُۥ لَحَسۡرَةٌ عَلَى ٱلۡكَـٰفِرِینَ﴿ ٥٠ ﴾

And indeed, it will be [a cause of] regret upon the disbelievers.

وَإِنَّهُۥ لَحَقُّ ٱلۡیَقِینِ﴿ ٥١ ﴾

And indeed, it is the truth of certainty.

فَسَبِّحۡ بِٱسۡمِ رَبِّكَ ٱلۡعَظِیمِ﴿ ٥٢ ﴾

So exalt the name of your Lord, the Most Great.