הלכה על תרומות 4:15

To separate the great tithe for the priest: That we were commanded to separate the priestly tithe from the grain and the wine and the oil and we give it to the priest. And this is what is called the great tithe. And about this is it stated (Deuteronomy 18:4), "The first of your grain, your wine and your oil, etc. shall you give him." And they, may their memory be blessed, said (Chullin 137b), that there is no measure for it from Torah writ. Rather, even one [grain of] wheat exempts a large pile. But the Sages said (Mishnah Terumot 4:3) to separate more. And they said that one who has a moderate eye separated one part in fifty; and they based the thing upon the language, terumah (tithe) - meaning to say, trei memeah (two of a hundred), which is one from fifty.