  The result of a search for the decay Bc+ -> Bs0 pi+ is presented, using the
Bs0 -> Ds- pi+ and Bs0 -> J/psi Phi channels. The analysis is based on a data
sample of pp collisions collected with the LHCb detector, corresponding to an
integrated luminosity of 1 fb-1 taken at a center-of-mass energy of 7 TeV, and
2 fb-1 taken at 8 TeV. The decay Bc+ -> Bs0 pi+ is observed with significance
in excess of five standard deviations independently in both decay channels. The
measured product of the ratio of cross-sections and branching fraction is
sigma(Bc+)/sigma(Bs0) x BR(Bc+ -> Bs0 pi+) = (2.37 \pm 0.31 (stat) \pm 0.11
(syst)^{+0.17}_{-0.13} (tau_Bc)) x 10^-3 in the pseudorapidity range 2 < eta(B)
< 5, where the first uncertainty is statistical, the second is systematic and
the third is due to the uncertainty on the Bc lifetime. This is the first
observation of a B meson decaying to another B meson via the weak interaction.
