  Rare leptonic decays of $B_{(s)}^0$ mesons are sensitive probes of New
Physics effects. A combination of the CMS and LHCb analyses on the search of
the rare decays $B_{s}^0 \rightarrow \mu^+\mu^-$ and $B^0 \rightarrow
\mu^+\mu^-$ is presented. The branching fractions of $B_{s}^0 \rightarrow
\mu^+\mu^-$ and $B^0 \rightarrow \mu^+\mu^-$ are measured to be
$\mathcal{B}(B_{s}^0 \rightarrow \mu^+\mu^-) = (2.8 \,^{+0.7}_{-0.6}) \times
10^{-9}$ and $\mathcal{B}(B^0 \rightarrow \mu^+\mu^-) = (3.9 \,^{+1.6}_{-1.4})
\times 10^{-10}$ respectively. A statistical significances of $6.2\,\sigma$ is
evaluated for $B_{s}^0 \rightarrow \mu^+\mu^-$ from the Wilks' theorem while a
significance of $3.0\, \sigma$ is measured for $B^0 \rightarrow \mu^+\mu^-$
from the Feldman-Cousins procedure.
