Sa'id bin Jubayr (may Allah have mercy on him) narrates from Sayyiduna Ibn Abbas (may Allah be pleased with them both) regarding the saying of Allah Almighty, "Indeed, We sent it down on the Night of Decree" [Surah al-Qadr: 1], that the Qur'an was sent down all at once to the lowest heaven on the Night of Decree, and it was at the place where the stars descend, and Allah Almighty would send it down as occasions required, one revelation after another. Allah Almighty says: They said, "Why was the Qur'an not sent down to him all at once?" [Surah al-Furqan: 32] "So that We may strengthen your heart thereby." [Surah al-Furqan: 32]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8521