أَمْ يَقُولُونَ افْتَرَاهُ قُلْ فَأْتُوا بِعَشْرِ سُوَرٍ مِثْلِهِ مُفْتَرَيَاتٍ وَادْعُوا مَنِ اسْتَطَعْتُمْ مِنْ دُونِ اللَّهِ إِنْ كُنْتُمْ صَادِقِينَ ﴿۱۳﴾ فَإِلَّمْ يَسْتَجِيبُوا لَكُمْ فَاعْلَمُوا أَنَّمَا أُنْزِلَ بِعِلْمِ اللَّهِ وَأَنْ لَا إِلَهَ إِلَّا هُوَ فَهَلْ أَنْتُمْ مُسْلِمُونَ ﴿۱۴﴾

﴾13﴿ Am yaqooloonaf taraahu qul faatoo bi'ashri Suwarim mislihee muftarayaatinw wad'oo manis tata'tum min doonil laahi in kuntum saadiqeen
﴾14﴿ Fa il lam yastajeeboo lakum fa'lamooo annamaaa unzilla bi'ilmil laahi wa al laaa ilaaha illaa Huwa fahal antum muslimoon

﴾13﴿ Do they say, He has fabricated this Quran himself, Say, "Then bring forth ten chapters like it, fabricated, and call upon whomever you can besides Allah, if you are truthful
﴾14﴿ So if they do not respond to your words, then know that this Qur'an has indeed been revealed with the knowledge of Allah, the Exalted, and that there is no deity worthy of worship except Allah. So, will you then accept His commandments

[13] This is a warning to those who deny the Qur'an and a demonstration of its miraculous nature by challenging them to produce ten surahs like it. It is said that at that time there were ten major opposing tribes, and one surah was requested from each tribe. Another view is that from the beginning up to this point in the Qur'an, ten surahs had been revealed, so the challenge refers to matching these.
Question: In Surah Yunus, the challenge is to produce one surah, while here it is ten — and this surah was revealed after Surah Yunus. So why does the challenge increase from less to more?
Answer (according to Mubarrad): In Surah Yunus, the challenge refers to matching the Qur'an in laws, promises and warnings, and truthful reports of the unseen — for that, even one surah is enough to prove the point. But in this surah, the challenge is purely about eloquence and rhetorical excellence, and for that, multiple surahs are needed to make the contrast and difference more apparent.
[14] In this verse, the truthfulness of the Qur'an is affirmed, and it explains the core message for which the Qur'an was revealed — that is, There is no god but Him (لَا إِلَهَ إِلَّا هُوَ). The word (لَكُمْ) in the plural indicates that even the believers are included in the challenge posed to oppose the Prophet (May Allah bless him and give him peace). (بِعِلْمِ اللَّهِ) means that the Qur'an contains the knowledge of Allah, and since it encompasses divine knowledge, no creation can match or oppose the knowledge of Allah. (وَأَنْ لَا إِلَهَ إِلَّا هُوَ) — this phrase is either connected to the knowledge of Allah, or to the phrase أَنَّمَا, and in both interpretations, it serves as evidence that the subject and essence of the Qur'an is Tawhid — the Oneness of Allah.