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