  A search is performed for the lepton number violating decay $B^{+}\to h^-
\mu^+ \mu^+$, where $h^-$ represents a $K^-$ or a $\pi^-$, using data from the
LHCb detector corresponding to an integrated luminosity of $36pb^{-1}$. The
decay is forbidden in the Standard Model but allowed in models with a Majorana
neutrino. No signal is observed in either channel and limits of $B(B^{+} \to
K^- \mu^+ \mu^+) < 5.4\times 10^{-8}$ and $B(B^{+} \to \pi^- \mu^+ \mu^+) <
5.8\times 10^{-8}$ are set at the 95% confidence level. These improve the
previous best limits by factors of 40 and 30, respectively.
