In the paper ``Analytic Continuation Of Chern-Simons Theory'' (arXiv:1001.2933) Witten postulates that hyperbolic volume of 3-dimensional manifold coincides with the value of the Chern-Simons functional of the hyperbolic connection (see section 5.3.4).

Let me state this more precisely.

Let $M$ be a three dimensional spin manifold. Consider a Riemannian metric $\rho$ on $M$ with constant negative curvature $-1$. The universal cover ($\tilde{M},\tilde{\rho}$) is isometric to the hyperbolic space (${H^3},\rho^{st}$). The fundamental group of $M$ acts on $H^3$ by isometries. It therefore defines a homomorphism $g:\pi_1(M)\to \text {Isom}(H^3)=\text {PSL}(2,{\mathbb{C}})$.

**Def** The *hyperbolic connection* $A_{\rho}$ on the trivial $PSL(2,\mathbb{C})$-bundle $E$ on $M$ is a flat connection with monodromy representation $g$.

**Rem** The inclusion $\text{SO}(3)\subset \text{PSL}(2,\mathbb{C})$ is a homotopy equivalence. Since $M$ has a spin structure we can lift $A_{\rho}$ to an $SL(2,\mathbb{C})$-connection.

**Def** The value of the *Chern-Simons functional* on an $\text{SL}(2,\mathbb{C})$-connection $A$ in the trivial bundle on $M$ is given by
\begin{equation}
CS(A):=\int_{M}tr[A,dA]+\frac{2}{3}tr[A,A\wedge A].
\end{equation}
Here $tr[\cdot,\cdot]$ is defined as follows:
\begin{equation}
tr[\cdot,\cdot]: \Omega^n(M, g)\otimes\Omega^m(M, g) \xrightarrow{\wedge} \Omega^{m+n} (M, g\otimes g) \xrightarrow{tr} \Omega^{m+n} (M,\mathbb{C}).
\end{equation}
Here $g$ is a simple Lie algebra and the trace over the last arrow is the standard non-degenerate invariant symmetric bilinear form on $g$.

**Rem** A gauge transformation $s \in \Omega^0(M,E)$ changes CS by an integer: $CS(A)-CS(s^*A)\in 2\pi \mathbb{Z}$.

Finally, what I'm seeking for is a reference for the formula (which seems to be well known) \begin{equation} 2\pi \text{ Im } \text{CS}(A_{\rho})=\text{Vol}_{\rho}. \end{equation}