From Sahih Muslim (Hadith 5923):
Jabir reported Allah's Messenger (ﷺ) 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
Reference: Muslim 5923