Allah almighty our creator says,
And this Qur’an is not a thing that could be composed without Allah’s Revelation: nay, it is the confirmation of what was revealed before it(Torah, Gospel), and full explanation of `the Book’.
There is no doubt about this that it is from the Lord of the universe.
Qur’an 10 [37]
Allah almighty our creator says,
Do they say that the Prophet has composed it himself? Tell them,
“If what you say be true, then produce one surah like this and you may call to your assistance anyone you can other than Allah.”
The fact is that they have denied (on mere conjecture) what was not within the reach of their knowledge nor had they, as yet, tasted its consequences.
Likewise, the people who went before them declared as false the Revelations, but behold, what was the end of those unjust people! Some of these people will believe in it and others will not, and your Lord knows these mischief-makers very well indeed.
If these people declare that you are an impostor say to them "I am responsible for my deeds and you are for yours: you are not accountable for what I do, and I am not for what you do.”
Qur’an 10 [38-41]
Allah almighty says,
The revelation of this book(Qur’an)is, without any doubt, from the Lord of the Worlds. Do the people say that this man(Prophet Muhammad) himself has forged it?
Nay, but it is the Truth from your Lord so that you may warn a people to whom no warner has come before you. Maybe they are guided aright.
Qur’an 32 (2-3)
Allah almighty says,
This is the Book of Allah(God): there is no doubt about it.
It is guidance to Godfearing people, who believe in the unseen, establish the Salats(Prayer) and expend (in Our way) out of what We have bestowed on them; who believe in the Book We have sent down to you (i.e. the Qur’an) and in the Books sent down before you, and firmly believe in the Hereafter.
Qur’an 2 (2-4)
Allah almighty challenges you,
And if you be in doubt whether the Book(Qur’an) We have sent down to Our Servant is from Us or not, then produce, at least, one Surah(Chapter) like this.
You may call all your associates to assist you and avail yourselves of the help of any one other than Allah.
If you are genuine in your doubt, do this. But if you do not do this, and you can never do this, then fear the Fire which has been prepared for the disbelievers and which shall have men and stones for fuel.
Qur’an 2(23-24)