We derive a general integral formula on an embedded hypersurface for general relativistic spacetimes. Suppose the hypersurface is foliated by two-dimensional compact `sections' . Then the formula relates the rate of change of the divergence of outgoing light rays integrated over under change of section to geometric (convexity and curvature) properties of and the energy-momentum content of the spacetime. We derive this formula using the Sparling - Nester - Witten identity for spinor fields on the hypersurface by appropriate choice of the spinor fields. We discuss several special cases which have been discussed in the literature before, most notably the Bondi mass loss formula.