Shulchan Arukh, Orach Chayim 220/שולחן ערוך·אורח חיים ר״כ

1Rectifying a dream and fasting about it. Contains two paragraphs. When one has a dream and is anxious about it, one can rectify the dream in front of three people who care about him. He can say to them: "I had a good dream" and they can say to him "it is a good dream and it will be well."

2It is good to fast to nullify a bad dream. Like fire on wood chips. Rem"a: Specifically, this applies to the same day and even on Shabbat.