Sahih Muslim | Book 29 | Hadith Number 5616

Narrated by Abu Qatada
Abu Qatada reported Allah's Messenger (may peace be upon him) as saying: A good vision is from Allah and a bad dream (hulm) is from the satan; so if one of you sees anything (in a dream which he dislikes, he should spit on his left side thrice and seek refuge with Allah from its evil, and then it will never harm him. Abu Salama said: I used to see dreams weighing more heavily upon me than a mountain; but since I heard this hadith I don't care for it (its burden).