Musannaf Ibn Abi Shayba 23:185

Abdullah bin Numayr narrated to us, from Yahya bin Sa'id, from Abu Salamah, from Abu Qatadah, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "The truthful vision is from Allah, and the dream is from Satan. So if one of you sees what he dislikes, let him spit to his left and seek refuge in Allah from its evil, for it will not harm him."