Metallic materials with kagome lattice structure are interesting because their electronic structures can host flat bands, Dirac cones, and van Hove singularities, resulting in strong electron correlations, nontrivial band topology, charge density wave (CDW), and unconventional superconductivity. Recently, kagome lattice compounds AV3Sb5 (A=K, Rb, Cs) are found to have intertwined CDW order and superconductivity. The origin of the CDW has been suggested to arise from Fermi-surface instabilities of van Hove singularity (saddle point) near the M points with weak electron-phonon coupling. Here we use neutron scattering experiments to demonstrate that the CDW order in CsV3Sb5 is associated with static lattice distortion and a sudden hardening of the B3u longitudinal optical phonon mode at the Brillouin zone boundary, thus establishing that the wave vector dependent electron-phonon coupling must also play an important role in the CDW order of AV3Sb5.