TT #129 · Parashas Shoftim · Third Aliyah · Passuk 18:03
And this shall be the due of the kohanim from the people, from those who slaughter the sacrifice, whether ox or sheep: he shall give the kohen the foreleg, the jaws, and the maw.
Torah Temimah points to Sifrei Devarim 165.
TT lemma: וְנָתַן לַכֹּהֵן.
English: ‘And he shall give to the kohen’ – this is a positive commandment.
Derash classifying the mitzvah (Sifrei) — the language creates a positive obligation to give.
Giving the three gifts to a kohen is a positive commandment incumbent on the slaughterer.