Al-Furqan 25:32

Waqala allatheena kafaroo lawlanuzzila AAalayhi alqur-anu jumlatan wahidatan kathalikalinuthabbita bihi fu-adaka warattalnahu tarteela

And those who disbelieve say, "Why was the Qur’ān not revealed to him all at once?" Thus [it is] that We may strengthen thereby your heart. And We have spaced it distinctly.

Said those who disbelieved, “Why has the Qur’ān not been revealed to him all at once?” (It has been sent down) in this way (i.e. in parts) so that We make your heart firm, and We revealed it little by little.