The most recent search for the Standard Model Higgs boson decaying to a pair of $\tau$ leptons is performed using proton-proton collision events at a centre-of-mass energy of 13 TeV, recorded by the CMS experiment at the LHC. The full 2016 dataset, corresponding to an integrated luminosity of $35.9~fb^{-1}$, has been analysed. The Higgs boson signal in the $\tau^{+}\tau^{-}$ decay mode is observed with a significance of 4.9 standard deviations, to be compared to an expected significance of 4.7 standard deviations. This measurement is the first observation of the Higgs boson decay into fermions by a single experiment.