Narrated Abu Qatada (Radi-Allahu 'anhu):

The Prophet (Sallallahu 'Alaihi Wa Sallam) said, "A good dream is from
Allah, and a bad or evil dream is from Satan; so if anyone of you has a bad
dream of which he gets afraid, he should spit on his left side and should
seek Refuge with Allah from its evil, for then it will not harm him."

Bukhari Vol. 4 : No. 513