Magnetars are neutron stars powered by their own magnetic energy and are believed to host the strongest magnetic fields in the Universe. The magnetar model was proposed to explain the sources of peculiar gamma-ray bursts, the soft gamma repeaters (SGRs), and then extended to interpret a subclass of X-ray pulsars, the anomalous X-ray pulsars (AXPs). Various observational properties of SGRs and AXPs are well consistent with such a picture, but a direct measurement of their surface magnetic field through the detection of a cyclotron line in their spectra has been searched for many years without success. I report on our recent discovery of a phase-variable absorption line in the X-ray spectrum of SGR 04181+5729 (Tiengo et al. 2013, Nature, 500, 312), which is the first narrow feature significantly detected in a magnetar spectrum and the most variable line ever observed in a neutron star. If interpreted as a proton cyclotron line, it implies the presence of a super-strong magnetic field in this object and constitutes one of the best confirmations for the magnetar model.