We report a detection of the baryon acoustic oscillation (BAO) feature in the
three-dimensional correlation function of the transmitted flux fraction in the
\Lya forest of high-redshift quasars. The study uses 48,640 quasars in the
redshift range $2.1\le z \le 3.5$ from the Baryon Oscillation Spectroscopic
Survey (BOSS) of the third generation of the Sloan Digital Sky Survey
(SDSS-III). At a mean redshift $z=2.3$, we measure the monopole and quadrupole
components of the correlation function for separations in the range
$20\hMpc