Majma al-Zawaid — Hadith 11505

Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Qur'an was sent down all at once to the lowest heaven in the month of Ramadan, on the Night of Decree (Laylat al-Qadr), and then from there it was sent down in separate portions.
Al-Tabarani recorded it in al-Mu'jam al-Awsat and al-Mu'jam al-Kabir; its chain contains Imran al-Qattan, whom Ibn Hibban and others declared thiqah (trustworthy), though there is weakness in him; the rest of its narrators are thiqat (trustworthy).