25:32
وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ٱلْقُرْءَانُ جُمْلَةً وَٰحِدَةً ۚ كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَ ۖ وَرَتَّلْنَـٰهُ تَرْتِيلًا
waqāla alladhīna kafarū lawlā nuzzila ʿalayhi l-qur'ānu jum'latan wāḥidatan kadhālika linuthabbita bihi fuādaka warattalnāhu tartīlan
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 disbelieve say: "Why is not the Qur’ân revealed to him all at once?" Thus (it is sent down in parts), that We may strengthen your heart thereby. And We have revealed it to you gradually, in stages. (It was revealed to the Prophet صلى الله عليه وسلم in 23 years.).