correct phrasing

I personally would insert "that" between "saying" and "'in'".

With the comma, it means that John has already replied to your post, in which reply he said that 'I' could be used in the sentence.

Without the comma, it means that John has already replied to your post, in which you said that 'I' could be used in the sentence.

Thanks, Contrex.

Your reply is very informative.