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International audience; We present the first observations of foreground Ly alpha forest absorption fromhigh-redshift galaxies, targeting 24 star-forming galaxies (SFGs) with z similar to 2.3-2.8 within a 5' x 14' region of the COSMOS field. The transverse sightline separation is similar to 2h(-1) Mpc comoving, allowing us to create a tomographic reconstruction of the three-dimensional (3D) Lya forest absorption field over the redshift range 2.20 \textless= z \textless= 2.45. The resulting map covers 6h(-1) Mpc x 14h(-1) Mpc in the transverse plane and 230h(-1) Mpc along the line of sight with a spatial resolution of approximate to 3.5h(-1) Mpc, and is the firsthigh-fidelity map of a large-scale structure on similar to Mpc scales at z \textgreater 2. Our map reveals significant structures with greater than or similar to 10h(-1) Mpc extent, including several spanning the entire transverse breadth, providing qualitative evidence for the filamentary structures predicted to exist in thehigh-redshift cosmic web. Simulated reconstructions with the same sightline sampling, spectral resolution, and signal-to-noise ratio recover the salient structures present in the underlying 3D absorption fields. Using data from other surveys, we identified 18 galaxies with known redshifts coeval with our map volume, enabling a direct comparison with our tomographic map. This shows that galaxies preferentially occupyhigh-density regions, in qualitative agreement with the same comparison applied to simulations. Our results establish the feasibility of the CLAMATO survey, which aims to obtain Ly alpha forest spectra for similar to 1000 SFGs over similar to 1deg(2) of the COSMOS field, in order to map out the intergalactic medium large-scale structure at \textless z \textgreater similar to 2.3 over a large volume (100h(-1) Mpc)(3).