Allah states that the idolators used to proclaim, "Why does not (Muhammad) bring an Ayah from his Lord,'' meaning, a miracle of their choice! They would sometimes say,
﴿لَن نُّؤْمِنَ لَكَ حَتَّى تَفْجُرَ لَنَا مِنَ الاٌّرْضِ يَنْبُوعًا﴾
("We shall not believe in you, until you cause a spring to gush forth from the ground for us.'') ﴿17:90﴾.
﴿قُلْ إِنَّ اللَّهَ قَادِرٌ عَلَى أَن يُنَزِّلٍ ءايَةً وَلَـكِنَّ أَكْثَرَهُمْ لاَ يَعْلَمُونَ﴾
(Say: "Allah is certainly able to send down a sign, but most of them know not.'') Certainly, Allah is able to send an Ayah (sign). But, He decided out of His wisdom to delay that, because if He sends an Ayah of their liking and they still do not believe, this will hasten their punishment as with the previous nations. Allah said in other Ayat,
﴿وَمَا مَنَعَنَآ أَن نُّرْسِلَ بِالاٌّيَـتِ إِلاَّ أَن كَذَّبَ بِهَا الاٌّوَّلُونَ وَءَاتَيْنَا ثَمُودَ النَّاقَةَ مُبْصِرَةً فَظَلَمُواْ بِهَا وَمَا نُرْسِلُ بِالاٌّيَـتِ إِلاَّ تَخْوِيفًا ﴾
(And nothing stops Us from sending the Ayat but that the people of old denied them. And We sent the she-camel to Thamud as a clear sign, but they did her wrong. And We sent not the signs except to warn, and to make them afraid (of destruction).) ﴿17:59﴾, and,
﴿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﴾.