There is no correct source for this quotation, because it originates in a folk tale, and therefore by definition has no author and no known source. The 1937 film has "Magic mirror on the wall" (followed by "who is the fairest one of all?" and, later in the film, "who now is the fairest one of all?"). The Grimms' version of the story has "Spieglein, Spieglein, an der Wand, Wer ist die Schönste im ganzen Land?" (literally: "Mirror, mirror, on the wall, who is the most beautiful in the whole countrhttps://en.wikiquote.org/wiki/Misquotations