Idolators ask to witness Miracles
`Ali bin Abi Talhah reported that Ibn `Abbas commented on Allah's statement,
قَالُواْ لَوْلاَ اجْتَبَيْتَهَا
(they say, "Why have you not brought it") "They say, `Why have you not received a miracle"', or, "Why have you not initiated or made it" Ibn Jarir reported that, `Abdullah bin Kathir said that Mujahid said about Allah's statement,
وَإِذَا لَمْ تَأْتِهِم بِـَايَةٍ قَالُواْ لَوْلاَ اجْتَبَيْتَهَا
(And if you do not bring them a miracle, they say: "Why have you not brought it") "They say, `Produce a miracle of your own."' Qatadah, As-Suddi, `Abdur-Rahman bin Zayd bin Aslam and Ibn Jarir agreed with this. Allah said next,
وَإِذَا لَمْ تَأْتِهِم بِـَايَةٍ
(And if you do not bring them an Ayah) a miracle or a sign. Similarly, Allah said,
إِن نَّشَأْ نُنَزِّلْ عَلَيْهِمْ مِّنَ السَّمَآءِ ءَايَةً فَظَلَّتْ أَعْنَـقُهُمْ لَهَا خَـضِعِينَ
(If We will, We could send down to them from the heaven a sign, to which they would bend their necks in humility.) 26:4 The pagans asked the Prophet, why did you not strive hard to bring us an Ayah (miracle) from Allah so that we witness it and believe in it. Allah said to him,
قُلْ إِنَّمَآ أَتَّبِعُ مَا يِوحَى إِلَىَّ مِن رَّبِّى
(Say: "I but follow what is revealed to me from my Lord.") I do not ask such things of my Lord. I only follow what He reveals and commands me. Therefore, if Allah sends a miracle, I will accept it. Otherwise, I will not ask for it unless He allows me. Certainly, Allah is Most Wise, the All-Knower. Allah next directs the servants to the fact that this Qur'an is the most powerful miracle, clearest evidence and most true proof and explanation, saying,
هَـذَا بَصَآئِرُ مِن رَّبِّكُمْ وَهُدًى وَرَحْمَةً لِّقَوْمٍ يُؤْمِنُونَ
(This (the Qur'an) is nothing but evidences from your Lord, and a guidance and a mercy for a people who believe.)
Tafsir Ibn Kathir (abridged, English).