מדרש על חולין 229:20

(Devarim 14:6) "And every beast that has split hooves": Because we find that Scripture construed "treifah" ("torn" [in Shemoth 22:30 "and flesh in the field treifah you shall not eat"]) as "treifoth" (i.e., varieties of treifah that are forbidden to all [even non-priests]), I will also construe "pasul" (unfit) as "p'sulim" (i.e., unfit for eating for all), e.g., one's plowing with an ox and an ass (together), or with (animals dedicated as) offerings. It is, therefore, written (to include these as permitted to be eaten) "the ox, the lamb of sheep, and the kid of goats shall you eat."