TT #160 · Parashas Shoftim · Third Aliyah · Passuk 18:05
For the Eternal your God has chosen him from all your tribes to stand and serve in the name of the Eternal, he and his sons, all the days.
Torah Temimah points to Sifrei Devarim 167.
TT lemma: כִּי בוֹ בָּחַר ה׳.
English: ‘For the Eternal your God has chosen him’ – therefore He gave him the gifts.
Derash linking election and gifts (Sifrei) — the chosen status justifies the portions assigned to the kohen.
Because Hashem chose the kohen from among the tribes, the people are obligated to support him with the designated gifts.