It is not a matter of if, but when
 وَقَالُواْ لَن نُّؤْمِنَ لَكَ حَتَّى تَفْجُرَ لَنَا مِنَ الأَرْضِ يَنبُوعًا
أَوْ تَكُونَ لَكَ جَنَّةٌ مِّن نَّخِيلٍ وَعِنَبٍ فَتُفَجِّرَ الأَنْهَارَ خِلالَهَا تَفْجِيرًا
أَوْ تُسْقِطَ السَّمَاء كَمَا زَعَمْتَ عَلَيْنَا كِسَفًا أَوْ تَأْتِيَ بِاللّهِ وَالْمَلآئِكَةِ قَبِيلاً
And they said: “We will not put faith in you (Muhammad) till you explode for us from the earth a spring.  Or that you have a garden of date palms and grapes, and cause rivers to gush forth therein abundantly.  Or that you drop the heaven (sky), as you have claimed, on us fragments, or bring Allah and the angels in person.”
The Glorious Quran: chapter 17, verses  90-92   (610 AD)
The leaders of Mekkah, who disbelieved in Prophet Mohammed as the last messenger from Allah to all mankind, were not satisfied with the Quran as a clear sign confirming his prophet hood.  Therefore, they asked him to perform other miracles, as stated in the above verses. Then they challenged him to hasten their punishment and to drop on them  fragments from the heaven (sky), as Allah had warned them in verse 34:9 on the previous poster “قَالُوا : فَأَسْقِطْ السَّمَاء عَلَيْنَا كِسَفًا كَمَا زَعَمْت أَنَّ رَبّك إِنْ شَاءَ فَعَلَ”.
Furthermore, verse 8:32 indicates that in their stubbornness and ignorance, rather than asking Allah to guide them to the truth, they challenged Him to hasten the punishment by raining down on them stones from the heaven (sky).  In verse 29:53 Allah responded by promising those who challenged Him and His messenger, that the punishment is certainly coming, but it will only come at an appointed time that He alone knows when. 
وَإِذْ قَالُواْ اللَّهُمَّ إِن كَانَ هَـذَا هُوَ الْحَقَّ مِنْ عِندِكَ فَأَمْطِرْ عَلَيْنَا حِجَارَةً مِّنَ السَّمَاء أَوِ ائْتِنَا بِعَذَابٍ أَلِيمٍ
And when they said: O Allah! If this is indeed the truth from You, then rain down on us stones from the heaven (sky) or bring us a painful punishment!
The Glorious Quran: chapter 8, verses  32   (610 AD)
وَيَسْتَعْجِلُونَكَ بِالْعَذَابِ وَلَوْلَا أَجَلٌ مُّسَمًّى لَجَاءهُمُ الْعَذَابُ وَلَيَأْتِيَنَّهُم بَغْتَةً وَهُمْ لَا يَشْعُرُونَ
And they hasten you with the punishment:  And had it not been for
 a term appointed, the punishment would have come to them: and it will certainly come to them without a warning while they perceive it not!
The Glorious Quran: chapter 29, verse  53   (610 AD)