  We present a measurement of the $CP$-violating weak mixing phase $\phi_s$
using the decay $\bar{B}^{0}_{s}\to D_{s}^{+}D_{s}^{-}$ in a data sample
corresponding to $3.0$ fb$^{-1}$ of integrated luminosity collected with the
LHCb detector in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV. An
analysis of the time evolution of the system, which does not constrain
$|\lambda|=1$ to allow for the presence of $CP$ violation in decay, yields
$\phi_s = 0.02 \pm 0.17$ (stat) $\pm 0.02$ (syst) rad, $|\lambda| =
0.91^{+0.18}_{-0.15}$ (stat) $\pm0.02$ (syst). This result is consistent with
the Standard Model expectation.
