Zevachim 44A

אם לא נאמרו חמורות, הייתי אומר על החמורות במיתה?! הא מטומאת שרץ קאתיא, ודיו לבא מן הדין להיות כנדון!

If the stringent case of impurity imparted by a corpse were not stated, but only the lenient case of the impurity of a creeping animal, would I say that the punishment in the stringent case is that of death at the hand of Heaven? Rather, the halakha in the case of impurity imparted by a corpse would be derived from the halakha in the case of the impurity of a creeping animal by means of an a fortiori inference, and it is sufficient for the conclusion that emerges from an a fortiori inference to be like its source, in this case that one is liable to be flogged for the violation of a prohibition, and no more.

אמר זעירי: קלות ⁠– טומאת שרץ, חמורות ⁠– טומאת מת. והכי קאמר: אילו נאמר טומאת שרץ ונאמר מעשר ותרומה, ולא נאמרה טומאת מת ⁠– הייתי אומר: [קלות] על הקלות בלאו, ועל החמורות במיתה;

Ze’eiri says: Indeed, the lenient case is referring to the impurity of a creeping animal, and the stringent case is referring to impurity imparted by a corpse. And this is what the baraita is saying: If the impurity of a creeping animal was stated, and it was stated that one who eats second tithe while impure with such impurity has violated a prohibition and one who partakes of teruma in that state is liable to be punished with death at the hand of Heaven, and the impurity imparted by a corpse was not stated in this context, I would say as follows: The lenient level of impurity, that of a creeping animal, with regard to food with lenient halakhot, second tithe, involves the violation of a prohibition, and with regard to food with stringent halakhot, teruma, it involves liability to be punished with death at the hand of Heaven.