A search for the rare decays ${B}_{s}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ and ${B}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ is performed in $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$, with a data sample corresponding to an integrated luminosity of $1.14\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, collected by the CMS experiment at the LHC. In both cases, the number of events observed after all selection requirements is consistent with expectations from background and standard-model signal predictions. The resulting upper limits on the branching fractions are $\mathcal{B}({B}_{s}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})l1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ and $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})l4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$, at 95% confidence level.
