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" (JA)
App reference (JA): Book 56, Hadith 8 (JA)
English reference (JA): Book 55, Hadith 395 (JA)
Arabic reference (JA): Book 56, Hadith 413 (JA)