A study of $B^{0}_{s} \to \eta_{c} \phi$ and $B^{0}_{s} \to \eta_{c}
\pi^{+}\pi^{-}$ decays is performed using $pp$ collision data corresponding to
an integrated luminosity of 3.0$\,\rm fb^{-1}$, collected with the LHCb
detector in Run~1 of the LHC. The observation of the decay $B^{0}_{s} \to
\eta_{c} \phi$ is reported, where the $\eta_{c}$ meson is reconstructed in the
$p\bar p$, $K^+K^-\pi^+\pi^-$, $\pi^+\pi^-\pi^+\pi^-$ and $K^+K^-K^+K^-$ decay
modes and the $\phi(1020)$ in the $K^+ K^-$ decay mode. The decay $B^{0}_{s}
\to J/\psi \phi$ is used as a normalisation channel. Evidence is also reported
for the decay $B^{0}_{s} \to \eta_{c} \pi^{+}\pi^{-}$, where the $\eta_{c}$
meson is reconstructed in the $p\bar p$ decay mode, using the decay $B^{0}_{s}
\to J/\psi \pi^+ \pi^-$ as a normalisation channel. The measured branching
fractions are \begin{eqnarray*} {\mathcal B (B^{0}_{s} \to \eta_{c} \phi)} &=&
\left(5.01 \pm 0.53 \pm 0.27 \pm 0.63 \right) \times 10^{-4} \,, \nonumber
\\ {\mathcal B (B^{0}_{s} \to \eta_{c} \pi^+ \pi^-)} &=& \left(1.76 \pm 0.59
\pm 0.12 \pm 0.29 \right) \times 10^{-4} \,, \end{eqnarray*} where in each case
the first uncertainty is statistical, the second systematic and the third
uncertainty is due to the limited knowledge of the external branching
fractions.