A magnetic resonance - in the sense of a dramatic change in spin excitation spectrum - has not been convincingly established in the archetypal high temperature cuprate (HTC) superconductor La2 xSrxCuO4 (LSCO). Our recent experiment using IN8 on optimally doped LSCO has revealed strong temperature dependence in the excitations at 18 meV at (0.5,0.5+d), which correspond to an incommensurate resonance. However, there is an optical phonon near 18 meV that complicates the extraction of pure magnetic intensity. Here, we propose to use polarized neutrons to isolate magnetic scattering from phonons and measure pure magnetic excitations through 18 meV below and above Tc. Our results should unambiguously reveal the incommensurate resonance in LSCO