  We discuss the origin of the neutron scattering peak at 41 meV observed in
YBa$_2$Cu$_3$O$_7$ below $T_c$. The peak may occur due to spin-flip electron
excitations across the superconducting gap which are enhanced by the
antiferromagnetic interaction between Cu spins. In this picture, the experiment
is most naturally explained if the superconducting order parameter has $s$-wave
symmetry and opposite signs in the bonding and antibonding electron bands
formed within a Cu$_2$O$_4$ bilayer.
