  We report on the observation of the accreting pulsar GRO J1008-57 performed
by Insight-HXMT at the peak of the source's 2017 outburst. Pulsations are
detected with a spin period of 93.283(1) s. The pulse profile shows double
peaks at soft X-rays, and only one peak above 20 keV. The spectrum is well
described by the phenomenological models of X-ray pulsars. A cyclotron resonant
scattering feature is detected with very high statistical significance at a
centroid energy of $E_{\rm cyc}=90.32_{-0.28}^{+0.32}$ keV, for the reference
continuum and line models, HIGHECUT and GABS respectively. Detection is very
robust with respect to different continuum models. The line energy is
significantly higher than what suggested from previous observations, which
provided very marginal evidence for the line. This establishes a new record for
the centroid energy of a fundamental cyclotron resonant scattering feature
observed in accreting pulsars. We also discuss the accretion regime of the
source during the Insight-HXMT observation.
