  The Cabibbo-suppressed semileptonic decay $B^{+}\to
p\overline{p}\mu^{+}\nu_{\mu}$ is observed for the first time using a sample of
$pp$ collisions corresponding to an integrated luminosity of 1.0, 2.0 and
1.7fb$^{-1}$ at centre-of-mass energies of 7, 8 and 13TeV, respectively. The
differential branching fraction is measured as a function of the
$p\overline{p}$ invariant mass using the decay mode $B^{+}\to J/\psi K^{+}$ for
normalisation. The total branching fraction is measured to be \begin{align*}
  \mathcal{B}(B^{+}\to p\overline{p}\mu^{+}\nu_{\mu}) = (5.27 ^{+0.23}_{-0.24}
\pm 0.21 \pm 0.15)\times 10^{-6}, \end{align*} where the first uncertainty is
statistical, the second systematic and the third is from the uncertainty on the
branching fraction of the normalisation channel.
