A search for the rare decays B 0 s → μ + μ − and B 0 → μ + μ − is performed at the LHCb experiment. The data analyzed correspond to an integrated luminosity of 1     fb − 1 of p p collisions at a center-of-mass energy of 7 TeV and 2     fb − 1 at 8 TeV. An excess of B 0 s → μ + μ − signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of B ( B 0 s → μ + μ − ) = ( 2.9 + 1.1 − 1.0 ) × 10 − 9 is obtained and an upper limit of B ( B 0 → μ + μ − ) < 7.4 × 10 − 10 at 95% confidence level is set. These results are consistent with the standard model expectations.