  Let $(N,g)$ be an $n$-dimensional complete Riemannian manifold with nonempty
boundary $\pt N$. Assume that the Ricci curvature of $N$ has a negative lower
bound $Ric\geq -(n-1)c^2$ for some $c>0$, and the mean curvature of the
boundary $\pt N$ satisfies $H\geq (n-1)c_0>(n-1)c$ for some $c_0>c>0$. Then a
known result (see \cite{LN}) says that $\sup_{x\in N}d(x,\pt N)\leq \frac
1c\coth^{-1}\frac{c_0}c$. In this paper, we prove that if the boundary $\pt N$
is compact, then the equality holds if and only if $N$ is isometric to the
geodesic ball of radius $\frac 1c\coth^{-1}\frac{c_0}c$ in an $n$-dimensional
hyperbolic space $\mathbb{H}^n(-c^2)$ of constant sectional curvature $-c^2$.
Moreover, we also prove an analogous result for manifold with nonempty boundary
and with $m$-Bakry-\'{E}mery Ricci curvature bounded below by a negative
constant.
