Simultaneous neutron scattering and thermal expansion measurements on the heavy-fermion superconductor URu2 Si2 under hydrostatic pressure of 0.67 GPa have been performed in order to detect the successive paramagnetic, hidden order, and large moment antiferromagnetic phases on cooling. The temperature dependence of the sharp low energy excitation at the wave vector Q0 = (1,0,0) shows clearly that this resonance is a signature of the hidden order state. In the antiferromagnetic phase, this resonance disappears. The higher energy excitation at the incommensurate wave vector Q1 = (1.4,0,0) persists in the antiferromagnetic phase but increases in energy.