  Inelastic neutron scattering has been used to determine the dynamic spin
fluctuations in an underdoped high temperature superconductor YBCO_{6.8} single
crystal. The magnetic resonance, that occurs around 40 meV in overdoped
samples, is shifted to a lower energy, E_r= 34 meV. A constant ratio, $E_r/ k_B
T_C= 4.9 \pm 0.2$, almost independent of the doping level, is found. According
to numerous theoretical approaches, this finding supports the idea that the
resonance energy is proportional (approximately twice) to the superconducting
gap.
