وَقَالَ الَّذِيْنَ كَفَرُوْا لَوْلَا نُزِّلَ عَلَيْهِ الْقُرْاٰنُ جُمْلَةً وَّاحِدَةً ۛ كَذٰلِكَ ۛ لِنُثَبِّتَ بِهٖ فُؤَادَكَ وَرَتَّلْنٰهُ تَرْتِيْلًا ( الفرقان: ٣٢ )
Wa qaalal lazeena kafaroo law laa nuzzila 'alaihil Quraanu jumlatanw waahidah; kazaalika linusabbita bihee fu'aadaka wa rattalnaahu tarteelaa (al-Furq̈ān 25:32)
And those who disbelieve say, "Why was the Quran not revealed to him all at once?" Thus [it is] that We may strengthen thereby your heart. And We have spaced it distinctly. (Al-Furqan  : 32)
1 Mufti Taqi Usmani
Said those who disbelieved, “Why has the Qur’ān not been revealed to him all at once?” (It has been sent down) in this way (i.e. in parts) so that We make your heart firm, and We revealed it little by little.