Sayyiduna Ibn Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whoever sees me in a dream should be certain that he has truly seen me, for Shaytan cannot take my form or appearance."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4193