Sahih Bukhari Volume 9, Book 87, Hadith Number 169.

Narrated By Abu Sa’id Al-Khudri : I heard Allah’s Apostle saying, “If anyone of you saw a dream which he liked, then that was from Allah, and he should thank Allah for it and tell it to others; but if he saw something else, i.e, a dream which he did not like, then that is from Satan and he should seek refuge with Allah from it and should not tell it to anybody for it will not harm him.”