The Quran was sent down all at once on Laylat al-Qadr to the lowest heaven

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Allah Almighty sent down the Noble Qur'an (all at once) to the lowest heaven on the Night of Decree (Laylat al-Qadr). Then whenever Allah Almighty wished to reveal anything of it, He revealed it, or when He wished to bring about some new event on earth, He brought it about.
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that concerning the saying of Allah Almighty, "Indeed, We sent it down on the Night of Decree" [Surah al-Qadr: 1], he said: The Noble Qur'an was sent down complete, all at once, on the Night of Decree to the lowest heaven, and it was arranged in the manner of the positions of the stars. Then Allah Almighty kept sending down parts of it to His Messenger (peace and blessings be upon him), one after another, as needed. The narrator says: The disbelievers said: "Why was the Qur'an not sent down to him all at once? Thus (it is), that We may strengthen your heart thereby, and We have revealed it in stages" [Surah al-Furqan: 32].
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Allah Almighty sent down the Noble Qur'an complete, all at once, to the lowest heaven on the Night of Decree; then after that it was sent down (gradually) over a period of twenty years: "And they do not bring you any example except that We bring you the truth and the best explanation" [Surah al-Furqan: 33]; and He also said: "And (it is) a Qur'an which We have divided (into parts) so that you may recite it to the people at intervals, and We have sent it down progressively" [Surah al-Isra: 106].
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that when revelation came down upon the Messenger of Allah (peace and blessings be upon him), none of us could bring himself to raise his gaze towards him (peace and blessings be upon him) until the revelation had ended.
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Noble Qur'an was separated from the Dhikr (the Preserved Tablet) and placed in the House of Honour (Bayt al-Izzah) in the lowest heaven. Then Jibril (peace be upon him) kept sending it down upon the Prophet (peace and blessings be upon him) in stages and in measured order.
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Disputing about the Qur'an is disbelief."