Aims:We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS). Methods: Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV). Results: The light curves of HS 1857+5144 display a sinusoidal variation with a period of P_orb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1 mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of M_wd≃ 0.6-1.0 {M}&sun; and T_wd≃ 70 000-100 000 K, respectively. The detection of He II 4686 absorption classifies the primary star of HS 1857+5144 as a DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be M_sec≃ 0.15-0.30 {M}&sun; and i≃ 45°-55°, respectively. Conclusions: We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase 105 yr ago. HS 1857+5144 will start mass transfer within or below the 2-3 h period gap.