Anas Radiyallahu 'Anhu reports that Rasulullah Sallallahu 'Alayhi Wasallam said:
"The one who sees me in a dream has really seen me, because the shaytaan cannot impersonate me". He also said: "A dream of a Muslim (conveyed through an angel) is a portion from the forty six portions of nubuwwah"
App reference: Book 56, Hadith 8
English reference: Book 55, Hadith 395
Arabic reference: Book 56, Hadith 413