Sahih Muslim
The Book of Vision (Kitab Al-Ruya)
Book Number: 029 / Hadith Number: 5638
Jabir reported Allah's Messenger (may peace be upon him) as saying: He who saw me in sleep in fact saw me, for it is not possible for the satan to appear in my form; and he also said: When any one of you sees a hulm he should not inform anyone, for it is a sort of vain sport of devil in the state of sleep.