Rashi on Numbers 18:12/רש"י על במדבר י״ח:י״ב

1ראשיתם THE FIRST FRUITS OF THEM [WHICH THEY SHALL GIVE UNTO THE LORD; THEM I HAVE GIVEN THEE] — This refers to the “T’rumah Gedolah” (the priest’s portion of the produce which is the first portion to be set aside from the produce; cf. Deuteronomy 18:4).