  Thus far, a detection of the Dirac or Weyl fermions in topological semimetals
remains often elusive, since in these materials conventional charge carriers
exist as well. Here, measuring a field-induced length change of the prototype
Weyl semimetal TaAs at low temperatures, we find that its $c$-axis
magnetostriction amounts to relatively large values whereas the $a$-axis
magnetostriction exhibits strong variations with changing the orientation of
the applied magnetic field. It is discovered that at magnetic fields above the
ultra-quantum limit, the magnetostriction of TaAs contains a linear-in-field
term, which, as we show, is a hallmark of the Weyl fermions in a material.
Developing a theory for the magnetostriction of noncentrosymmetric topological
semimetals and applying it to TaAs, we additionally find several parameters
characterizing the interaction between the relativistic fermions and elastic
degrees of freedom in this semimetal. Our study shows how dilatometry can be
used to unveil Weyl fermions in candidate topological semimetals.
