THERE is a growing consensus that the emergence of quasars at high redshifts is related to the onset of galaxy formation(1), suggesting that the detection of concentrations of gas accompanying such quasars should provide flues about the early history of galaxies. Quasar companions have been recently identified at redshifts up to z approximate to 3 (refs 2-4). Here we report observations of Lyman-alpha emission (a tracer of ionized hydrogen) from the companion to a quasar at z = 4.7, corresponding to a time when the Universe was less than ten per cent of its present age. We argue that most of the emission arises in a gaseous nebula that has been photoionized by the quasar, but an additional component of continuum light-perhaps quasar light scattered from dust in the companion body, or emission from young stars within the nebula-appears necessary to explain the observations. These observations may be indicative of the first stages in the assembly of galaxy-sized structures.