Musnad Ahmad 30:3603

Sulayman ibn Dawud—who is Abu Dawud al-Tayalisi—narrated to us, Imran—meaning al-Qattan—narrated to us, from Qatadah, from Abu Maymunah, from Abu Hurayrah, that the Messenger of Allah, peace and blessings of Allah be upon him, said regarding the Night of Decree (Laylat al-Qadr): "It is the twenty-seventh—or twenty-ninth—night. Indeed, the angels on that night are more numerous on the earth than the number of pebbles."