'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".
App reference: Book 56, Hadith 1
English reference: Book 55, Hadith 389
Arabic reference: Book 56, Hadith 406