Musnad Abu Ya'la al-Mawsili 9:3

Qasim ibn Abi Shaybah narrated to us, Abu Usamah narrated to us, from Sadaqah ibn Abi Imran, from Awn ibn Abi Juhayfah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sees me in a dream, it is as if he has seen me while awake, for the Shaytan cannot impersonate me."