  Modified gravitational wave (GW) propagation is a generic phenomenon in
modified gravity. It affects the reconstruction of the redshift of coalescing
binaries from the luminosity distance measured by GW detectors, and therefore
the reconstruction of the actual masses of the component compact stars from the
observed (`detector-frame') masses. We show that, thanks to the narrowness of
the mass distribution of binary neutron stars, this effect can provide a clear
signature of modified gravity, particularly for the redshifts explored by third
generation GW detectors such as Einstein Telescope and Cosmic Explorer.
