So if someone posts a message, and someone responds to it, we call that a 'reply'.
However, I am struggling to think of a good phrase for the situation where you have the reply, and you want to see what it is a reply to? What would we call the entity that it is a reply to? "See message" doesn't seem to semantically capture it.
e.g. something like "antecedent", but that seems overly wordy.