Musnad Ahmad 1007:77

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Abu Salamah, who said: I used to experience intensity from dreams, except that I would not be wrapped [in anxiety/fear] until Abu Qatadah narrated to me that he heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "The true dream is from Allah, and the nightmare is from the Shaytan. So if one of you has a dream he dislikes, let him spit to his left three times, and let him seek refuge in Allah from the Shaytan, for it will not harm him."