  We prove a CR version of the Obata's result for the first eigenvalue of the
sub-Laplacian in the setting of a compact strictly pseudoconvex pseudohermitian
three dimensional manifold with non-negative CR-Panietz operator which
satisfies a Lichnerowicz type condition. We show that if the first positive
eigenvalue of the sub-Laplacian takes the smallest possible value then, up to a
homothety of the pseudohermitian structure, the manifold is the standard
Sasakian three dimensional unit sphere.
