Authentic Supplication Number 36.

It is reported of the Messenger of Allah  that he said: “If one of you sees a Ru’ya which he hates then let him spit to his left 3 times, let him seek protection in Allah from the shaytan 3 times, and let him switch the side of his body that he was sleeping on.”

Reporters.
Reported by Muslim and in The Authentic of Good Sayings as #40.