{And if they were to see a fragment from the sky} - al-kisaf is a piece - {falling, they would say, 'Clouds heaped up,'} one upon another. This is because He said in Surat Saba': {If We willed, We could cause the earth to swallow them or let fall upon them fragments from the sky}. They said to the Prophet: 'We will not believe in you until you cause the sky to fall upon us as a fragment, as you claimed.' So Allah revealed: {And if they were to see a fragment from the sky falling, they would say, 'Clouds heaped up,'} meaning: and they did not believe.
﴿وَإِنْ يَرَوْا كِسْفًا مِنَ السَّمَاءِ﴾ والكِسْفُ: القطْعة ﴿سَاقِطًا يَقُولُوا سَحَابٌ مركوم﴾ بعضُه على بعض، وَذَلِكَ أَنه قَالَ فِي سُورَة سبأ: ﴿إِنْ نَشَأْ نَخْسِفْ بِهِمُ الأَرْضَ أَوْ نُسْقِطْ عَلَيْهِمْ كِسَفًا من السَّمَاء﴾ فَقَالُوا للنَّبِي: لن نؤمن لَك حَتَّى تُسْقِطَ السَّمَاءَ كَمَا زَعَمْتَ عَلَيْنَا كسفاً؛ فَأنْزل اللَّه: ﴿وَإِنْ يَرَوْا كِسْفًا مِنَ السَّمَاءِ سَاقِطًا يَقُولُوا سَحَابٌ مركوم﴾ أَي: وَلم يُؤمنُوا.
