Sahih Muslim | Book 29 | Hadith Number 5638

Narrated by Jabir
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.