$B_{(s)}^0\to {\mu }^{+}{\mu }^{-}$ at CMS

The search for the rare \BMuMu and \BsMuMu decays in pp collisions at $\sqrt s = 7$GeVand$\sqrt s = 8$~GeV, collected at the LHC in 2011 and 2012, is briefly reviewed. The data analyzed by CMS correspond to a total integrated luminosity of 5 and 20 $\mathrm{f}{\mathrm{b}}^{-1}$, respectively. The time-integrated average branching fraction $\left<{\mathcal B }( { B^{0}{s}\rightarrow \mu^+\mu^-} )\right>$hasbeenmeasuredtobe$(3.0 ^{+1.0}{-0.9})\times 10^{-9}$ in accordance with the Standard Model predictions, while an upper limit $\left<{ \mathcal B }( B^{0}\rightarrow \mu^+\mu^-)\right> < 1.1\times 10^{-9}) $ has been placed on the other investigated decay at 95% CL. A preliminary combination with the results from LHCb is also presented for both channels, and prospects for the future are briefly discussed.