Sahih Al Bukhari – Book of Medicine

Volumn 007, Book 071, Hadith Number 643. 
—————————————– 
Narated By Abu Qatada

Radi-Allahu 'anhu: I heard the Prophet

Sallallahu 'Alaihi Wa 'Aalehi Wa Sallam saying , "A good dream is from Allah, and a bad dream is from Satan. So if anyone of you sees (in a dream) something he dislikes, when he gets up he should blow thrice (on his left side) and seek refuge with Allah from its evil for then it will not harm him."