Results are presented from a search for the rare decays B0s → μ+μ- and B0 → μ+μ- in pp collisions at √s = 7 and 8 TeV, with data samples corresponding to integrated luminosities of 5 and 20 fb-1, respectively, collected by the CMS experiment at the LHC. An unbinned maximum-likelihood fit to the dimuon invariant mass distribution gives a branching fraction B(B0s → μ+μ-) = (3.0+1.0-0.9) x 10-9, where the uncertainty includes both statistical and systematic contributions. An excess of B0s → μ+μ- events with respect to background is observed with a significance of 4.3 standard deviations. For the decay B0 → μ+μ- an upper limit of B(B0 → μ+μ- ) < 1.1 x 10-9 at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model.