Sahih Al Bukhari – Book of Beginning Of Creation
Volumn 004, Book 054, Hadith Number 513.
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Narated By Abu Qatada : The Prophet said, "A good dream is from Allah, and a bad or evil dream is from Satan; so if anyone of you has a bad dream of which he gets afraid, he should spit on his left side and should seek Refuge with Allah from its evil, for then it will not harm him."