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.