  The recurrent hard pulsating X-ray transient \S was observed with \B on March
19, when the source was at a 2--10 keV flux level of $\sim$310~mCrab. We report
on the high energy spectrum of the source, concentrating on cyclotron resonant
scattering features. The spectrum is strongly pulse phase dependent and
absorption features are detected at virtually all phases. In particular, four
absorption-like features at 12.74 keV, 24.16 keV, 35.74 keV, and 49.5 keV are
observed in the descending edge of the main peak of the pulse profile. The
ratios between the centroid energies of the lines with respect to the first are
1:(1.9):(2.8):(3.9). These values are close to the harmonic relation expected
from cyclotron resonant scattering in a strong magnetic field, when
relativistic effects are taken into account. The equivalent widths of the
second, third, and fourth harmonics are found to be larger than that of the
first harmonic, confirming the key role of two-photon processes in the spectral
formation. These results provide the first evidence for {\it four harmonically
spaced lines} in the spectrum of an accreting X-ray pulsar, yielding the
clearest confirmation to date of their magnetic origin.
