A reviewer asked for a citation that the sample mean is a consistent and unbiased estimator of the expected value and therefore converges towards the expected value. I know I can easily do the calculations myself based on the variance of the sample mean etc. However, since the reviewer specifically asked for a citation, I am not sure a proof instead would be acceptable. Also, I don't like adding proofs that have almost nothing to do with the actual subject of the article.
In most books I looked at, this fact is either only provided for the normal distribution (i.e. sample mean is an estimator of the parameter $\mu$), or included as an exercise.
Is anybody aware of a citation that I could use to support the claim that the sample mean is an unbiased estimator for any distribution (assuming of course expected value exists and variance is finite).