Commentary for Deuteronomy 18:4

ראשית דגנך THE FIRST-FRUIT [ALSO] OF THY CORN … [THOU SHALT GIVE HIM] — This refers to the heave-offering. Scripture, however, does not state any minimum quantity, but our Rabbis fixed a quantity for it from a sixtieth to a fortieth: a benevolent eye (a generous person) gives one fortieth of the crop, a niggard at least one sixtieth, a person of average generosity one fiftieth. They found a support in Scripture that one should not give less than one sixtieth, because it is said, (Ezekiel 45:13) “[This is the heave-offering which ye shall offer:] the sixth part of an ephah of an homer of wheat”. Now the sixth part of an ephah is a half Seah; if therefore you give half a Seah as Terumah from a Kor (which is another term for a Homer), that is a sixtieth part, for a Kor is thirty Seahs (Talmud Yerushalmi Terumot 4:3).

One sixth of an eiphah (וששיתם האיפה). This denotes a sixth.

V. 4. ראשית דגנך וגו׳, die תרומה, auch בכורים und חלה, die alle ראשית heißen.