  A great deal of evidence has recently been gathered in favor of the picture
that Soft Gamma Repeaters and Anomalous X-Ray Pulsars are powered by
ultra-strong magnetic fields (B > 10^{14} G; i.e. magnetars). Nevertheless,
present determination of the magnetic field in such magnetar candidates has
been indirect and model dependent. A key prediction concerning magnetars is the
detection of ion cyclotron resonance features, which would offer a decisive
diagnostic of the field strength. Here we present the detection of a 5 keV
absorption feature in a variety of bursts from the Soft Gamma Repeater SGR
1806-20, confirming our initial discovery (Ibrahim et al. 2002) and
establishing the presence of the feature in the source's burst spectra. The
line feature is well explained as proton cyclotron resonance in an ultra-strong
magnetic field, offering a direct measurement of SGR 1806-20's magnetic field
(B ~ 10^{15} G) and a clear evidence of a magnetar. Together with the source's
spin-down rate, the feature also provides the first measurement of the
gravitational redshift, mass and radius of a magnetar.
