We report on the results obtained from the search for leptonic decays $\mathrm{B^0_s}\to \mu^+\mu^-$ and $\mathrm{B^0}\to \mu^+\mu^-$ in pp collisions at $\sqrt{s}=7$ TeV and 8 TeV, collected at CMS during 2011 and 2012 data-taking, for a total of 25 fb$^{-1}$ of integrated luminosity. We measured the branching fraction $\mathcal{B}$($\mathrm{B^0_s}\to \mu^+\mu^-$)=$(3.0^{+1.0}_{-0.9})\times 10^{-9}$ and we observed an excess of $\mathrm{B^0_s}\to \mu^+\mu^-$ events with a significance of 4.3 standard deviations. We measured the upper limit for the branching fraction of the $\mathrm{B^0}\to \mu^+\mu^-$, $\mathcal{B}$($\mathrm{B^0}\to \mu^+\mu^-$)<1.1 $\times 10^{-9}$, at 95% confidence level. We also report on the combined analysis of LHCb and CMS results and present the first observation of the $\mathrm{B^0_s}\to \mu^+\mu^-$ decay. The branching fractions of this decay as well as of the $\mathrm{B^0}\to \mu^+\mu^-$ decay are presented.