Corollary 8.3 in: Richard D. Canary: A covering theorem for hyperbolic 3-manifolds and its applications. In: Topology. Bd. 35, Nr. 3, 1996, S. 751–778, (Digitalisat (PDF; 2,5 MB)).
Albert Marden: The geometry of finitely generated Kleinian groups. In: Annals of Mathematics. Series 2, Bd. 99, Nr. 3, 1974, S. 383–762, doi:10.2307/1971059.