Musnad Ahmad 1007:128

Yahya ibn Sa'id narrated to us, from Yahya ibn Sa'id, who said: I heard Abu Salamah ibn 'Abd al-Rahman, [who] heard Abu Qatadah, that he heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "The true dream is from Allah, and the nightmare is from Satan. So if one of you sees something he dislikes, let him spit to his left three times, and let him seek refuge in Allah from its evil, for it will not harm him."