Lyman alpha blobs (LABs) are spatially extended nebulae of emission in the Lyα line of hydrogen, seen at high redshifts, and most commonly found in the dense environment of star-forming galaxies. The origin of Lyman alpha emission in the LABs is still unclear and under debate. Proposed powering sources generally fall into two categories: (1) photoionization, galactic super-winds/outflows, resonant scattering of Lyman alpha photons from starbursts or active galactic nuclei (AGNs) and (2) cooling radiation from cold streams of gas accreting onto galaxies. Here we analyse the gas kinematics within a LAB providing rare observational evidence for infalling gas. This is consistent with the release of gravitational accretion energy as cold streams radiate Lyman alpha photons. It also provides direct evidence for possible cold streams feeding the central galaxies. The infalling gas is not important by mass but hints at more than one mechanism to explain the origin of the extended Lyman alpha emission around young galaxies. It is also possible that the infalling gas may represent material falling back to the galaxy from where it originated, forming a galactic fountain.