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