You shall watch the matzot
Ibn Ezra on Exodus 12:17:1:
ושמרתם. הנה צוה להיות המצות שמורים מימות הקציר
My translation, which I hope is accurate:
"Here, we see the commandment that the matzot must be guarded from the days of the harvesting"
If my translation is correct, Ibn Ezra states how long the matzot themselves must be guarded; he doesn't state that the wheat has to be guarded. I also don't see how Ibn Ezra derives this as being a Mitzvah.
Thanks to @Ze'ev's comment and link to [this source],1 Shulchan Aruch appears to state that the main reason deals with chimutz - when and how the dough becomes chametz. There is a debate as to whether chametz concerns begin from the time that the wheat is ground or it starts earlier, from the time it is harvested.
OK, I got that. But my question is why the debate at all? Where from the Torah's verses does anyone derive that there even needs to be any type of guarding from harvesting? It appears that this is not just a chumra, but, the Shulchan Aruch makes this a requirement to eat only shmurah matzot at the seder!
How do they derive this as a requirement from the Torah verse, or any other place?
Where and why did this extra stringency arise?