Sahih Bukhari Volume 009, Book 087, Hadith Number 114.

Narrated By Abu Sa'id Al-Khudri : The Prophet said, "If anyone of you sees a dream that he likes, then it is from Allah, and he should thank Allah for it and narrate it to others; but if he sees something else, i.e., a dream that he dislikes, then it is from Satan, and he should seek refuge with Allah from its evil, and he should not mention it to anybody, for it will not harm him."