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