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We observe a narrow peaking structure in the $pK^{-}$ invariant-mass spectrumnear the $\Lambda\eta$ threshold. The peak is clearly seen in 1.5 millionevents of $\Lambda_{c}^{+} \rightarrow p K^{-} \pi^{+}$ decay using the$980~{\rm fb^{-1}}$ data sample collected by the Belle detector at the KEKBasymmetric-energy $e^{+}e^{-}$ collider. We try two approaches to explain thisstructure: as a new resonance and as a cusp at the $\Lambda\eta$ threshold. Thebest fit is obtained with a coherent sum of a Flatt$\rm{\acute{e}}$ functionand a constant background amplitude with the $\chi^2/{\rm n.d.f}=257/243$ ($p =0.25$), while the fits to Breit-Wigner functions are unfavored by more than7$\sigma$. The best fit explains the structure as a cusp at the $\Lambda\eta$threshold, and the obtained parameters are consistent with the known propertiesof $\Lambda(1670)$. The observation gives the first identification of athreshold cusp in hadrons from the spectrum shape.

Observation of a Threshold Cusp at the $\Lambda\eta$ Threshold in the  $pK^{-}$ Mass Spectrum with $\Lambda_{c}^{+} \rightarrow p K^{-} \pi^{+}$  Decays