But they say, "[The revelation is but] a mixture of false dreams; rather, he has invented it; rather, he is a poet. So let him bring us a sign just as the previous [messengers] were sent [with miracles]."
Al-Bukhari recorded that `Abdur-Rahman bin Yazid said that `Abdullah said, "Banu Isra'il, Al-Kahf, Maryam, Ta Ha and Al-Anbiya' - they are among the earliest and most beautiful Surahs and they are my treasure."
بِسْمِ اللَّهِ الرَّحْمَـنِ الرَّحِيمِ
In the Name of Allah, the Most Gracious, the Most Merciful.
The Hour is at hand but People are heedless
This is a warning from Allah of the approach of the Hour, and that people are heedless of it, i.e., they are not working for it or preparing for it. An-Nasa'i recorded that Abu Sa`id reported from the Prophet :
بَلْ قَالُوا أَضْغَاثُ أَحْلَامٍ (they said, [ The Further Qur'an is ] a mixture of hotch-potch dreams. - 21:5.) Dreams having an element of personal and shaitanic thoughts are called أَضْغَاثُ أَحْلَامٍ. That is why this term (أَضْغَاثُ أَحْلَامٍ ) has been translated as "hotch-potch dreams". What it means is that in the first place the unbelievers called the Qur'an as magic, then they described it as a collection of disturbed dreams, and then they said it was a forgery and fabrication against Allah Ta’ ala to call it His words; and finally they said that he ﷺ was a poet and the Qur'an represented his poetic compositions.
فَلْيَأْتِنَا بِآيَةٍ (So let him bring to us a sign - 21:5) It means that the unbelievers demanded from the Holy Prophet ﷺ certain specific miracles to be shown as a proof of his being a real Prophet. In response to this demand, Allah Ta` ala said in this verse that the same demand for miracles was also made by the people in the past from other prophets, and when their request was met, it was of no avail. They did not submit to Allah even after witnessing the miracles of their choice. And Allah has decreed that people who do not submit to Him even after they have seen the miracle of their choice are subjected to His wrath even in this world and are destroyed. Allah, in His Divine Mercy and in view of the honoured position of the Holy Prophet ﷺ had granted to his Ummah immunity from the terrible punishment which is the inevitable fate of those people who defy the Will of Allah. It was not, therefore, considered desirable to show them miracles of their choice because if they, like the earlier people, persisted in their unbelief even after seeing those miracles, they too would invite the wrath of Allah Ta` ala.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 4–5
Every man who is living in this world is nearer to death than life. In this respect he is virtually on the verge of coming face to face with his Day of Reckoning. But man is so reckless that he does not pay heed to any sort of reminder from God, whether given through a prophet or anyone else. He ignores the advice of the preacher of truth and dismisses it as something uttered by ‘a mere man’. When in Makkah the Prophet Muhammad started calling upon people to God by means of the Quran. The divine discourse enshrined therein moved people’s hearts. This became a serious matter for the Makkan leaders, because this jeopardized their leadership. The Quran urged people to accept the unity of God, whereas the great leaders of Makkah had maintained their supremacy on the basis of polytheism. In order to divert the attention of the people, they started spreading the notion that the effectiveness of the Prophet Muhammad’s message did not derive from its being the word of God; the force behind it was not that of Truth but of magic. It was all mere human rhetoric and was in no way to be considred a divine discourse. Albeit taking the name of God, those who indulged in such talk did not firmly believe that God was seeing and hearing them. Had they had real faith in God’s being all-knowing, that is, His being One who knows the unseen—they would never have uttered such insincere words.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.