وَقَالَ ٱلَّذِينَ كَفَرُواْ لَوۡلَا نُزِّلَ عَلَيۡهِ ٱلۡقُرۡءَانُ جُمۡلَةٗ وَٰحِدَةٗۚ كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَۖ وَرَتَّلۡنَٰهُ تَرۡتِيلٗا

The faithless say, ‘Why has not the Quran been sent down to him all at once?’ So it was, that We may strengthen your heart with it, and We have recited it [to you] in a measured tone.