Isidore of Pelusium (died about A.D. 449), Letter 100 — to Syros the Reader (Against the Novatians)(translated by Clive Sweeting)
Say to the disciple of Novatian’s pride: why are you foolishly boasting as if [you were] clean? Why are you pretending that you are sinless? Why deny the (fault) common to nature? Isaiah declares himself unclean; David knows that every man is a liar and that all were conceived and carried in the womb in sin. God Himself knows that human beings are devotedly attached to evil and require only the mercy of divine kindness- and do you arrogantly boast of being clean? Either then give over lying or from what you are doing be exposed as a laughing-stock or indeed mightily shameful.(source)
I think the expression "common to nature" is particularly interesting, since the opponents of original sin seem to be fond of allegedly that man's nature was not affected by Adam's sin. I should point out further that the source of this nature is not explained in this short letter. So, it would be a stretch to conclude from this short letter that Isidore of Pelusium had as well-developed an idea of original sin as Augustine did. Still, the statements are interesting, particularly as an expression of Eastern rejection of Pelagian errors.