Musnad Ahmad 31:659

Surayj narrated to us, Ibn Wahb narrated to us, from ‘Amr ibn al-Harith, that Darraj Abu al-Samh narrated to him, from Abu al-Haytham, from Abu Sa’id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The truest of dreams are those at the time of the dawns (al-as-har)."