﴿ وَإِن يَرَوْا۟ كِسْفًۭا مِّنَ ٱلسَّمَآءِ سَاقِطًۭا يَقُولُوا۟ سَحَابٌۭ مَّرْكُومٌۭ ﴾

“AND YET, if they [who refuse to see the truth] were to see part of the sky falling down, they would [only] say, “[It is but] a mass of clouds!””

Please check ayah 52:49 for complete tafsir.