TT sequence · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:19
And it shall be, when the Eternal your God gives you rest from all your enemies round about, in the land that the Eternal your God is giving you as an inheritance to possess it, that you shall blot out the memory of Amalek from under the heavens; you shall not forget.
Torah Temimah points to Sanhedrin 20b (and parallel baraita).
TT lemma: תִּמְחֶה אֶת זֵכֶר עֲמָלֵק.
תָּנוּ רַבָּנַן: שָׁלֹשׁ מִצְוֹת נִצְטַוּוּ יִשְׂרָאֵל בִּכְנִיסָתָן לָאָרֶץ: לְהַעֲמִיד לָהֶם מֶלֶךְ, וּלְהַכְרִית זַרְעוֹ שֶׁל עֲמָלֵק, וּלְבָנוֹת לָהֶם בֵּית הַבְּחִירָה.
English: Our Rabbis taught: Three commandments were given to Yisrael upon their entry into the Land: to appoint a king for themselves, to wipe out the seed of Amalek, and to build for themselves the Chosen House (the Temple).
On the daf / in context: Sanhedrin 20b places the obligation to eradicate Amalek among the three foundational mitzvos that apply specifically after entering Eretz Yisrael, in sequence with the appointment of a king and the building of the Beis HaMikdash.
Derash identifying the three entry-into-the-Land commandments (baraita in Sanhedrin) — the verse supplies the third of the triad.
Upon settling the Land, Yisrael are commanded three things: appoint a king, eradicate Amalek, and build the Temple. The blotting out of Amalek is one of the foundational mitzvos of the Land.