Surah Al-Furqan (The Criterion), Verse 32

وَقَالَ الَّذِیْنَ كَفَرُوْا لَوْلَا نُزِّلَ عَلَیْهِ الْقُرْاٰنُ جُمْلَةً وَّاحِدَةً كَذٰلِكَ لِنُثَبِّتَ بِهٖ فُؤَادَكَ وَرَتَّلْنٰهُ تَرْتِیْلًا ۟

Wa qaalal lazeena kafaroo law laa nuzzila 'alaihil Qur'aanu jumlatanw waahidah; kazaalika linusabbita bihee fu'aadaka wa rattalnaahu tarteelaa

“Those who reject Faith say: "Why is not the Qur'an revealed to him all at once? Thus (is it revealed), that We may strengthen thy heart thereby, and We have rehearsed it to thee in slow, well-arranged stages, gradually.”

Quran 25:32 (Yusuf Ali Translation)