[en] In a recent experiment of deep inelastic neutrino-nucleus scattering, it was observed that there is almost no correlation between produced mesons and slow protons when the multiplicity of the former is small while a positive correlation emerges as it becomes larger. We show that this is a direct manifestation of nonvanishing formation time. The correlation is calculated using an appropriate parametrization of semi-inclusive quark fragmentation functions. Comparison with data indicates that the proper formation time is 0.4-0.6 fm/c. (author)