The Quraysh claimed They can produce Something similar to the Qur'an

Allah tells;

وَإِذَا تُتْلَى عَلَيْهِمْ آيَاتُنَا قَالُواْ قَدْ سَمِعْنَا لَوْ نَشَاء لَقُلْنَا مِثْلَ هَـذَا إِنْ هَـذَا إِلاَّ أَسَاطِيرُ الأوَّلِينَ ﴿٣١﴾

And when Our Ayat are recited to them, they say: "We have heard (the Qur'an); if we wish we can say the like of this.

Allah describes the disbelief, transgression, rebellion, as well as misguided statements that the pagans of Quraysh used to utter when they heard Allah's Ayat being recited to them, قَدْ سَمِعْنَا لَوْ نَشَاء لَقُلْنَا مِثْلَ هَـذَا ("We have heard (the Qur'an); if we wish we can say the like of this).''

They boasted with their words, but not with their actions. They were challenged several times to bring even one chapter like the Qur'an, and they had no way to meet this challenge. They only boasted in order to deceive themselves and those who followed their falsehood.

It was said that;

An-Nadr bin Al-Harith, may Allah curse him, was the one who said this, according to Sa`id bin Jubayr, As-Suddi, Ibn Jurayj and others.

An-Nadr visited Persia and learned the stories of some Persian kings, such as Rustum and Isphandiyar. When he went back to Makkah, He found that the Prophet was sent from Allah and reciting the Qur'an to the people. Whenever the Prophet would leave an audience in which An-Nadr was sitting, An-Nadr began narrating to them the stories that he learned in Persia, proclaiming afterwards, "Who, by Allah, has better tales to narrate, I or Muhammad?''

When Allah allowed the Muslims to capture An-Nadr in Badr, the Messenger of Allah commanded that his head be cut off before him, and that was done, all thanks are due to Allah.

The meaning of, إِلاَّ أَسَاطِيرُ الأوَّلِين (. ..tales of the ancients) is that;

the Prophet has plagiarized and learned books of ancient people, and this is what he narrated to people, as they claimed.

This is the pure falsehood that Allah mentioned in another Ayah,

وَقَالُواْ أَسَـطِيرُ الاٌّوَّلِينَ اكْتَتَبَهَا فَهِىَ تُمْلَى عَلَيْهِ بُكْرَةً وَأَصِيلاً

قُلْ أَنزَلَهُ الَّذِى يَعْلَمُ السِّرَّ فِى السَّمَـوَتِ وَالاٌّرْضِ إِنَّهُ كَانَ غَفُوراً رَّحِيماً

And they say: "Tales of the ancients, which he has written down:, and they are dictated to him morning and afternoon.''

Say: "It (this Qur'an) has been sent down by Him (Allah) Who knows the secret of the heavens and the earth. Truly, He is Oft-Forgiving, Most Merciful.'' (25:5-6)

for those who repent and return to Him, He accepts repentance from them and forgives them.