وَقالَ الَّذينَ كَفَروا لَولا نُزِّلَ عَلَيهِ القُرآنُ جُملَةً واحِدَةً ۚ كَذٰلِكَ لِنُثَبِّتَ بِهِ فُؤادَكَ ۖ وَرَتَّلناهُ تَرتيلًا

The disbelievers question, “Why wasn’t the Quran revealed to him all at once?” It was thus, to strengthen your heart, that We revealed it in stages.