  We present the first clear identification and highly accurate measurement of
the intra-shell transition $1s2p\, ^3P_2 \to 1s2s\, ^3S_1$ of He-like uranium
performed via X-ray spectroscopy. The present experiment has been conducted at
the gas-jet target of the ESR storage ring in GSI (Darmstadt, Germany) where a
Bragg spectrometer, with a bent germanium crystal, and a Ge(i) detector were
mounted. Using the ESR deceleration capabilities, we performed a differential
measurement between the $1s2p\, ^3P_2 \to 1s2s\, ^3S_1$ He-like U transition
energy, at 4510 eV, and the $1s^22p\ ^2P_{3/2} \to 1s^22s\, ^2S_{1/2}$ Li-like
U transition energy, at 4460 eV. By a proper choice of the ion velocities, the
X-ray energies from the He- and Li-like ions could be measured, in the
laboratory frame, at the same photon energy. This allowed for a drastic
reduction of the experimental systematic uncertainties, principally due to the
Doppler effect, and for a comparison with the theory without the uncertainties
arising from one-photon QED predictions and nuclear size corrections.
