  A search for the rare decays Bs->mu+mu- and B0->mu+mu- is performed using
data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron
Collider. The data samples comprise 1.1 fb^-1 of proton-proton collisions at
sqrt{s} = 8 TeV and 1.0 fb^-1 at sqrt{s}=7 TeV. We observe an excess of Bs ->
mu+ mu- candidates with respect to the background expectation. The probability
that the background could produce such an excess or larger is 5.3 x 10^-4
corresponding to a signal significance of 3.5 standard deviations. A
maximum-likelihood fit gives a branching fraction of BR(Bs -> mu+ mu-) =
(3.2^{+1.5}_{-1.2}) x 10^-9, where the statistical uncertainty is 95% of the
total uncertainty. This result is in agreement with the Standard Model
expectation. The observed number of B0 -> mu+ mu- candidates is consistent with
the background expectation, giving an upper limit of BR(B0 -> mu+ mu-) < 9.4 x
10^-10 at 95% confidence level.
