  Starting from the three-band $p-d$ Hubbard Hamiltonian we derive an effective
$t-J$ model including electron-phonon interaction of quasiparticles with
optical phonons. Within the effective Hamiltonian we analyze the influence of
electronic correlations and electron-phonon interaction on the dynamical spin
susceptibility in layered cuprates. We find a huge isotope effect on the
resonance peak in the magnetic spin susceptibility, ${Im}\chi({\bf q},\omega)$,
seen by inelastic neutron scattering. It results from both the electron-phonon
coupling and the electronic correlation effects taken into account beyond
random phase approximation(RPA) scheme. We find at optimal doping the isotope
coeffiecient $\alpha_{res} \approx 0.35$ which can be further tested
experimentally.
