  This paper proves a positive energy-momentum theorem for oriented Riemannian
3-manifolds that are asymptotic to a standard hyperbolic slice in anti de
Sitter space-time. Analogously to the original Witten's proof in the
asymptotically flat case, this result relies on spinorial methods. We also give
a rigidity theorem: if the energy-momentum is degenerate (in a certain sense)
then our 3-manifold can be isometrically embedded in anti de Sitter.
