  We report $^{121/123}$Sb nuclear quadrupole resonance (NQR) and $^{51}$V
nuclear magnetic resonance (NMR) measurements on kagome metal CsV$_3$Sb$_5$
with $T_{\rm c}=2.5$ K. Both $^{51}$V NMR spectra and $^{121/123}$Sb NQR
spectra splits after charge density wave (CDW) transition, which demonstrates a
commensurate CDW order. Coexistence of high temperature phase and CDW order
phase between $91$ K and $94$ K manifest it is a first order phase transition.
At low temperature, EFG fluctuations diminish and magnetic fluctuations become
dominant. Superconductivity emerges in the charge order state. Knight shift
decrease below $T_{\rm c}$ and $1/T_{1}T$ shows a Hebel-Slichter coherence peak
just below $T_{\rm c}$, indicating that CsV$_3$Sb$_5$ is an $s$-wave
superconductor.
