We study the non-linear evolution of the curvature perturbations during matter dominated era .
We show that regardless of the origin of the primordial perturbation , the Bardeen potential and curvature receive sizable contributions from the classical non-linear evolution effects , and quantify them exactly .
On the super-horizon scales we have squeezed peak of the bispectrum with magnitude , in terms of the local non-linear parameters of Bardeen curvature , 1 / 6 \leq f _ { NL } \lesssim 19 / 15 , and of Bardeen potential , -1 / 4 \leq f _ { NL } \lesssim 7 / 5 , depending on the configuration of momenta .
On the sub-horizon scales the bispectrum show equilateral shape , and can serve as a potential probe of general relativity .