'Abdullah bin Mas'ud Radiyallahu 'Anhu reports that Rasulullah Sallallahu 'Alayhi Wasallam said:
"The one that has seen me in one's dream, has really seen me, as the shaytaan cannot impersonate me". (FR)
App reference (FR): Book 56, Hadith 1 (FR)
English reference (FR): Book 55, Hadith 389 (FR)
Arabic reference (FR): Book 56, Hadith 406 (FR)