Sahih Bukhari
Interpretation of Dreams
Volume: 009 / Book Number: 087 / Hadith Number: 133
Narrated Abu Qatada Al-Ansari:
(a companion of the Prophet and one of his cavalry men) "I heard Allah's Apostle saying, "A good dream is from Allah, and a bad dream is from Satan; so, if anyone of you had a bad dream which he disliked, then he should spit on his left and seek refuge with Allah from it, for it will not harm him."