Rashi on Exodus 17:14/רש"י על שמות י״ז:י״ד

1כתב זאת זכרון WRITE THIS FOR A MEMORIAL, that Amalek came to wage war against Israel before any other nation.

2ושים באזני יהושע AND PUT INTO THE EARS OF JOSHUA, who will bring Israel into the Holy Land, that he must command Israel to pay him his recompense. Here it was intimated to Moses (by the mere statement that Joshua should in some future time carry out the injunction to destroy Amalek) that Joshua would bring Israel into the Holy Land (cf. (Mekhilta).

3כי מחה אמחה BECAUSE I WOULD UTTERLY BLOT OUT [THE REMEMBRANCE OF AMALEK] — the reason why I admonish you thus is because (כי) I desire to blot it out.