Otto Stern Quote
A simple calculation shows that from the classical theory follows that we should find a broadening of the beam with the maximum intensity on the place of the beam without field. However, from the quantum theory follows that we should find there no intensity at all, and deflected molecules on both sides. The beam should split up in two beams corresponding to the two orientations of the magnet. The experiment decided in favor of the quantum theory.
the conclusion of the historical Stern-Gerlach experiment, in The Method of Molecular Rays, Nobel Lecture, December 12, 1946.