Musnad Ahmad 26:12

Ishaq—who is al-Azraq—narrated to us, Sufyan narrated to us, from Abu Ishaq, from Abu al-Ahwas, from Abdullah ibn Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sees me in a dream has indeed seen me, for it is not appropriate for the devil to impersonate my likeness."