Musnad Ahmad 1007:9

Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from Abu Salamah, who said: I used to see dreams that would make me ill, except that I would not be wrapped [in a blanket], until I met Abu Qatadah and mentioned that to him. He narrated to me from the Messenger of Allah (peace and blessings of Allah be upon him) that he said: "The true dream is from Allah, and the bad dream is from the Shaytan. So whoever sees a dream he dislikes, let him not tell it to anyone, and let him spit to his left three times, and let him seek refuge in Allah from its evil; for it will not harm him." Sufyan said on another occasion: "For he will not see anything he dislikes."