A search is performed for a spin-0 boson, ϕ, produced in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV, using prompt ϕ → μ+μ− decays and a data sample corresponding to an integrated luminosity of approximately 3.0 fb−1 collected with the LHCb detector. No evidence is found for a signal in the mass range from 5.5 to 15 GeV. Upper limits are placed on the product of the production cross-section and the branching fraction into the dimuon final state. The limits are comparable to the best existing over most of the mass region considered and are the first to be set near the ϒ resonances.