  We show that the recently observed spin resonance modes in heavy-fermion
superconductors CeCoIn$_5$ and CeCu$_2$Si$_2$ are magnetic excitons originating
from superconducting quasiparticles. The wave vector ${\bf Q}$ of the resonance
state leads to a powerful criterion for the symmetry and node positions of the
unconventional gap function. The detailed analysis of the superconducting
feedback on magnetic excitations reveals that the symmetry of the
superconducting gap corresponds to a singlet $d_{x^2-y^2}$ state symmetry in
both compounds. In particular this resolves the long-standing ambiguity of the
gap symmetry in CeCoIn$_5$. We demonstrate that in both superconductors the
resonance peak shows a significant dispersion away from ${\bf Q}$ that should
be observed experimentally. Our results suggest a unifying nature of the
resonance peaks in the two heavy-fermion superconductors and in layered
cuprates.
