the disbelievers say why has not the entire quran been sent down to this man all at nce? well this has been done to impress it deeply on your mind and we have sent it down piecemeal by degrees.
وقال الذين كفروا لولا نزل عليه القرآن جملة واحدة كذلك لنثبت به فؤادك ورتلناه ترتيلا