وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ٱلْقُرْءَانُ جُمْلَةً وَٰحِدَةً ۚ كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَ ۖ وَرَتَّلْنَـٰهُ تَرْتِيلًا
Wa qaalal lazeena kafaroo law laa nuzzila 'alaihil Quraanu jumlatanw waahidah; kazaalika linusabbita bihee fu'aadaka wa rattalnaahu tarteelaa
The disbelievers also say, ‘Why was the Quran not sent down to him all at once?’ We sent it in this way to strengthen your heart [Prophet]; We gave it to you in gradual revelation.
And those who deny say, "Why was the Qur'an not sent down to him all at once?" It is so — that we may steady your heart with it, and we have recited it in measured stages.