  Whistler waves are thought to play an essential role in the dynamics of
collisionless shocks. We use the magnetospheric multiscale (MMS) spacecraft to
study whistler waves around the lower hybrid frequency, upstream of 11
quasi-perpendicular super-critical shocks. We apply the 4-spacecraft timing
method to unambiguously determine the wave vector $\mathbf{k}$ of whistler
waves. We find that the waves are oblique to the background magnetic field with
a wave-normal angle between $20^{\circ}$ and $42^{\circ}$, a wavelength around
100 km which is close to the ion inertial length. We also find that
$\mathbf{k}$ is predominantly in the same plane as the magnetic field and the
normal to the shock. By combining this precise knowledge of $\mathbf{k}$ with
high-resolution measurements of the 3D ion velocity distribution we show that a
reflected ion beam is in resonance with the waves, opening up the possibility
for wave-particle interaction between the reflected ions and the observed
whistlers. The linear stability analysis of a system mimicking the observed
distribution, suggests that such a system can produce the observed waves.
