  Motivated by the iron-based superconductors, we develop a self-consistent
electronic theory for the itinerant spin excitations in the regime of
coexistence of the antiferromagnetic stripe order with wavevector ${\bf Q}_{1}
= (\pi,0)$ and $s^{+-}$ superconductivity. The onset of superconductivity leads
to the appearance of a {\em magnetic} resonance near the wavevector ${\bf
Q}_{2} = (0,\pi)$ where magnetic order is absent. This resonance is isotropic
in spin space, unlike the excitations near ${\bf Q}_{1}$ where the magnetic
Goldstone mode resides. We discuss several features which can be observed
experimentally.
