  We study the experimental $DK$ invariant mass spectra of the reactions $B^+
\to \bar{D}^0 D^0 K^+$, $B^0 \to D^- D^0 K^+$ (measured by the BaBar
Collaboration) and $B_s \to \pi^+ \bar{D}^0 K^-$ (measured by the LHCb
Collaboration), where an enhancement right above the threshold is seen. We show
that this enhancement is due to the presence of $D^\ast_{s0}(2317)$, which is a
$DK$ bound state in the $I(J^P) = 0(0^+)$ sector. We employ a unitarized
amplitude with an interaction potential fixed by heavy meson chiral
perturbation theory. We obtain a mass $M_{D^\ast_{s0}} = 2315^{+12}_{-17}\
^{+10}_{-5}\ \text{MeV}$, and we also show, by means of the Weinberg
compositeness condition, that the $DK$ component in the wave function of this
state is $P_{DK} = 70^{+4}_{-6}\ ^{+4}_{-8} \%$, where the first (second) error
is statistical (systematic).
