Al-Furqaan 32

وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ٱلْقُرْءَانُ جُمْلَةً وَٰحِدَةً كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَ وَرَتَّلْنَٰهُ تَرْتِيلًا ﴿٣٢﴾

Waqala allatheena kafaroo lawla nuzzila AAalayhi alquranu jumlatan wahidatan kathalika linuthabbita bihi fuadaka warattalnahu tarteelan

And those who disbelieve say: "Why is not the Quran 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 SAW in 23 years.). The Noble Quran