Tullio Levi-Civita, The Absolute Differential Calculus (calculus of Tensors) (1926), Part II, The Fundamental Quadratic Form and the Absolute Differential Calculus, Chapter VI, Covariant differentiation; invariants and differential parameters; locally geodesic co-ordinates, 11. Locally geodesic (or locally Cartesian) co-ordinates, p.164 [google books]