A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechanism for superconductivity. For iron pnictide superconductors, the s±-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies (E≤2Δ) below Tc. On the other hand, the s++-pairing symmetry expects a broad spin excitation enhancement at an energy above 2Δ below Tc. Although the resonance has been observed in iron pnictide superconductors at an energy below 2Δ consistent with the s±-pairing symmetry, the mode has also been interpreted as arising from the s++-pairing symmetry with E≥2Δ due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E=7 meV in SC NaFe0.935Co0.045As (Tc=18 K). On warming towards Tc, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitation spectra within the s± and s++-pairing symmetries, we conclude that the ground-state resonance in NaFe0.935Co0.045As is only consistent with the s± pairing, and is inconsistent with the s++-pairing symmetry.