Musnad Ahmad 1007:48

Abu al-Mughirah and Muhammad ibn Mus'ab narrated to us, they said: al-Awza'i narrated to us, Yahya narrated to me, from 'Abdullah ibn Abi Qatadah, from his father, that the Prophet, peace and blessings of Allah be upon him, said: "Indeed, the righteous vision is from Allah, and the dream (nightmare) is from Satan. So if one of you has a dream that he fears, let him spit to his left three times, and let him seek refuge in Allah from Satan, for it will not harm him."