Neutral Hydrogen ( HI ) provides a very important fuel for star formation , but is difficult to detect at high redshift due to weak emission , limited sensitivity of modern instruments , and terrestrial radio frequency interference ( RFI ) at low frequencies . We the first attempt to use gravitational lensing to detect HI line emission from three gravitationally lensed galaxies behind the cluster Abell 773 , two at redshift of 0.398 and one at z = 0.487 , using the Green Bank Telescope . We find a 3 \sigma upper limit for a galaxy with a rotation velocity of 200 km s ^ { -1 } is M _ { HI } = 6.58 \times 10 ^ { 9 } and 1.5 \times 10 ^ { 10 } M _ { \odot } at z = 0.398 and z = 0.487 . The estimated HI masses of the sources at z = 0.398 and z = 0.487 are a factor of 3.7 and \sim 30 times lower than our detection limits at the respective redshifts . To facilitate these observations we have used sigma clipping to remove both narrow- and wide-band RFI but retain the signal from the source . We are able to reduce the noise of the spectrum by \sim 25 \% using our routine instead of discarding observations with too much RFI . The routine is most effective when \sim 10 \% of the integrations or fewer contains RFI . These techniques can be used to study HI in highly magnified distant galaxies that are otherwise too faint to detect .