Sayyiduna Ibn Abbas (may Allah be pleased with them both) says, regarding "So I swear by the setting places of the stars" [Surah al-Waqi'ah: 75]: "The Qur'an was sent down all at once to the lowest heaven, then after that it was sent down to the Prophet (peace and blessings be upon him) in portions."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12373