Musannaf Ibn Abi Shayba 21:424

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