Transferring covariance matrix to an output file in ROOT

I am working on a project that requires me to fit data points to an exponential function. I am using the ROOT framework for the finding the best-fit parameters. I have done that part and I am exporting the best-fit values into a text file along with some descriptive statistics like errors on the fitted parameters, the chi-squared and reduced chi-squared values etc. I am also trying to transfer a covariance matrix to the output file but I am not able to do it. So, how do I transfer the covariance matrix to an output file in ROOT?