Penalty enforcement FED

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I guess what I really want to know is...if you have a two yard gain on first down and then tack on a five yard face-mask penalty, why is it still first down? If the offense gets credit for the two yard gain, shouldn't it be second down? I've been told you have to "replay the down", but you're not replaying the down if they've gotten the credit for the two yards. Several people have told me this is true, but I have yet to get a rule / reference for it.