  We report the first detection of the Galactic Centre massive black hole,
Sgr~A*, in the far infrared. Our measurements were obtained with PACS on board
the \emph{Herschel} satellite at $100~\mathrm{\mu m}$ and $160~\mathrm{\mu m}$.
While the warm dust in the Galactic Centre is too bright to allow for a direct
detection of Sgr~A*, we measure a significant and simultaneous variation of its
flux of $\Delta F_{\nu\widehat{=}160 ~\mathrm{\mu m}} =
(0.27\pm0.06)~\mathrm{Jy}$ and $\Delta F_{\nu\widehat{=}100 ~\mathrm{\mu m}}=
(0.16\pm0.10)~\mathrm{Jy}$ during one observation. The significance level of
the $160 ~\mathrm{\mu m}$ band variability is $4.5\sigma$ and the corresponding
$100 ~\mathrm{\mu m}$ band variability is significant at $1.6\sigma$. We find
no example of an equally significant false positive detection. Conservatively
assuming a variability of $25\%$ in the FIR, we can provide upper limits to the
flux. Comparing the latter with theoretical models we find that 1D RIAF models
have difficulties explaining the observed faintness. However, the upper limits
are consistent with modern ALMA and VLA observations. Our upper limits provide
further evidence for a spectral peak at $\sim 10^{12} ~ \mathrm{Hz}$ and
constrain the number density of $\gamma \sim 100$ electrons in the accretion
disk and or outflow.
