and the disbelievers say why was the quran not sent down to him all at once? this is how we sent it in stages in order to strengthen your heart with it and we read it slowly in stages.
وقال الذين كفروا لولا نزل عليه القرآن جملة واحدة كذلك لنثبت به فؤادك ورتلناه ترتيلا