Sforno on Exodus 17:14/ספורנו על שמות י״ז:י״ד

1כתוב זאת זכרון בספר, a reference to the portion in Deuteronomy 25,17-19.

2ושים באזני יהושע, make sure that Joshua remembers all this. Moses carried out this command by building the altar and offering up the prayer declaring G’d as his flag.

3מחה אמחה את זכר עמלק, not only the people but even their livestock. We know this from the instructions given by the prophet Samuel to King Sha-ul in Samuel 15,3 where the king is ordered to destroy utterly all the possessions of Amalek.