Abu Hurayrah Radiyallahu 'Anhu reports that Rasulullah Sallallahu 'Alayhi Wasallam said:
"The one who sees me in one's dream, has actually seen me, because the shaytaan cannot imitate my person". (IT)
App reference (IT): Book 56, Hadith 2 (IT)
English reference (IT): Book 55, Hadith 390 (IT)
Arabic reference (IT): Book 56, Hadith 407 (IT)