It is in <cite authors="Menger, K.">_Menger, K._, [**Untersuchungen über allgemeine Metrik.**](https://doi.org/10.1007/BF01448840), Math. Ann. 100, 75-163 (1928). [EuDML:159284](https://eudml.org/doc/159284). [JFM 54.0622.02](https://zbmath.org/54.0622.02).</cite> A modern exposition can be found in <cite authors="Bowers, John C.; Bowers, Philip L.">_Bowers, John C.; Bowers, Philip L._, [**A Menger redux: embedding metric spaces isometrically in Euclidean space**](https://doi.org/10.4169/amer.math.monthly.124.7.621), Am. Math. Mon. 124, No. 7, 621-636 (2017). [Zbl 1391.51007](https://zbmath.org/1391.51007).</cite>