How sound is the Hadith: "He who gives a fatwa on a matter or interprets a dream without knowledge is like one who falls from the sky to the earth and lands in a bottomless well, even if he were to hit the truth"? And what is the expiation for someone who used to interpret dreams before knowing this?

The aforementioned hadith (definition) has not been found in the recognized books of hadith. Rather, it was transmitted from "Musnad al-Rabi' ibn Habib," a book that is criticized and whose unique narrations are not to be accepted. Al-Albani described it as laden with denounced and false hadiths. Al-Rabi' is an unknown figure, not mentioned in the well-known books of biographical evaluation (rijal), nor is his Musnad found in the books of hadith. No specific expiation is required for one who interprets a dream out of ignorance of its principles; rather, he is obligated to repent.