Abdullah bin Abbas reported,
The Quran was sent down all at once to the lowest heaven on Laylat al-Qadr, and after that it was sent down over twenty years. "And they do not come to you with an example but that We bring you the truth and the best explanation" [Furqan 25:33], "And it is a Quran which We have separated by intervals that you might recite it to the people over a prolonged period. And We have sent it down progressively" [Isra 17:106].
Azami 668
Sahihعن ابن عباس قال
أنزل القرآن جملة واحدة إلى السماء الدنيا في ليلة القدر، أنزل بعد ذلك عشرين سنة: {ولا يأتونك بمثل إلا جئناك بالحق وأحسن تفسيرا} [سورة الفرقان: ٣٣]، {وقرآنا فرقناه لتقرأه على الناس على مكث ونزلناه تنزيلا} [سورة الإسراء: ١٠٦].
Nasai narrated it in Al-Kubra (7935), and Hakim (2/222), both through the route of Dawud bin Abu Hind, from Ikrima, from Ibn Abbas, and he mentioned it. Hakim said, "This hadith has a sahih isnad."