We report the result of a search for the decay $B^{-}tomu^{-}bar u_mu$. The signal events are selected based on the presence of a high momentum muon and the topology of the rest of the event showing properties of a generic $B$-meson decay, as well as the missing energy and momentum being consistent with the hypothesis of a neutrino from the signal decay. We find a 2.4 standard deviation excess above background including systematic uncertainties, which corresponds to a branching fraction of ${cal B}(B^{-}tomu^{-}bar u_mu) =(6.46 pm 2.22 pm 1.60 )times10^{-7}$ or a frequentist 90% confidence level interval on the $B^{-}tomu^{-}bar u_mu$ branching fraction of $[2.9, 10.7]times 10^{-7}$. This result is obtained from a $711 text{fb}^{-1}$ data sample that contains $772 times 10^6$ $Bbar{B}$ pairs, collected near the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider.
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