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".
App reference: Book 56, Hadith 2
English reference: Book 55, Hadith 390
Arabic reference: Book 56, Hadith 407