If you have a bad dream in your sleep, you should spit three times on the left side, then say three times -

أَعُوْذُ بِاللَّهِ مِنَ الشَّيْطَانِ الرَّجِيمِ

A'oodhu billaahi minash-Shaytaanir-rajeem

I seek refuge in Allah from Satan the outcast. One need to change sides in their sleep. It is forbidden to tell this dream in front of anyone. Even if you have good dreams, you don't have to tell anyone. But it can be said in front of a close friend or in front of wise people. Narrated Abu Qatadah (RA): The Prophet (ﷺ) said: A good dream is from Allah. And bad dreams are from the devil. So whoever has a good dream, let him reveal it to the one he loves. And if anyone has a bad dream, let him seek refuge in Allah from its harm and from the evil of Satan, and spit three times to the left. Don't reveal the dream to anyone. Then it will not be able to harm him. [1] Jarir (RA) said, Rasulullah (ﷺ) said, when one of you has a dream that he dislikes, he should spit three times to his left. And he seeks refuge in Allah from Satan three times and changes sides. [2] Abu Hurairah (RA) said: The Messenger of Allah (ﷺ) said: If a person has a bad dream, he should get up and pray two rak'ahs. [3]

Reference: [1] Bukhari: 7044 [2] Muslim: 6041 [3] Sahih (Albani). Tirmidhi: 2280