Sahih Bukhari Volume 9, Book 87, Hadith Number 124.

Narrated By Abu Qatada : The Prophet said, “A good dream is from Allah, and a bad dream is from Satan. So whoever has seen (in a dream) something he dislike, then he should spit without saliva, thrice on his left and seek refuge with Allah from Satan, for it will not harm him, and Satan cannot appear in my shape.”