# Calculating covariance of parameter estimation from Hessian

I need a covariance of parameter estimation for reliability estimation. According to the this post, I know the approximation of the Hessian can be calculated as

$$\sigma^2 H^{-1} = C$$

where H is Hessian, C is the covariance and sigma is the variance of residual. I am trying to understand where it came from and how it is derived but I could not find a related material anywhere. Anyone knows a good reading material regarding this?