  A search for the rare decays $B^0_s\rightarrow \mu^+\mu^-$ and
$B^0\rightarrow \mu^+\mu^-$ is performed with the LHCb experiment using
1.1\,fb$^{-1}$ of data collected at $\sqrt s=8$\,TeV$ and 1.0\,fb$^{-1}$ of
data collected at $\sqrt s=7$\,TeV. An excess of $B^0_s\rightarrow \mu^+\mu^-$
candidates with respect to the background expectations is observed with a
statistical significance of 3.5 standard deviations. A branching fraction of
${\cal BR}(B^0_s\rightarrow \mu^+\mu^-) =(3.2^{+1.5}_{-1.2}) \times 10^{-9}$ is
measured with an unbinned maximum likelihood fit. The measured branching
fraction is in agreement with the expectation from the Standard Model. The
observed number of $B^0\rightarrow \mu^+\mu^-$ candidates is consistent with
the background expectation and an upper limit on the branching fraction of
${\cal BR}(B^0\rightarrow \mu^+\mu^-) < 9.4\times10^{-10}$ is obtained.
