Sura al-Furqān 25:32

وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ٱلْقُرْءَانُ جُمْلَةً وَٰحِدَةً كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَ وَرَتَّلْنَٰهُ تَرْتِيلًا

wa-qāla lladhīna kafarū lawlā nuzzila ʿalayhi l-qurʾānu jumlatan wāḥidatan ka-dhālika li-nuthabbita bihi fuʾādaka wa-rattalnāhu tartīlan

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.