Musnad Abu Ya'la al-Mawsili 42:384

Zuhayr narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Abdullah ibn Wahb narrated to me, from Amr ibn al-Harith, from Darraj, from Abu al-Haytham, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The truest of dreams are those at the time of the predawn (al-as-har)."