Tosefta к Бамидбар 26:51

Said Rabbi Yosei, how do we know that terumah is one in fifty? As it is said (Num. 31:30), "And from the half-share of the Children of Israel you are to take one, withheld from the fifty," [and therefore] even that which I (i.e., God) apportioned to you in another place, behold, just as there it is one in fifty, so too here it is one in fifty. How do we know that if we take terumah and [only] one in sixty come into our hands, that our terumah is [valid] terumah? As it is said (Ezek. 45:13), "This is the contribution you shall make: One-sixth of an ephah from every chomer of wheat and one-sixth of an ephah from every chomer of barley" (an ephah is one-tenth of a chomer, and thus a sixth of an ephah is one sixtieth of a chomer). Rabbi Yishmael [son of Rabbi Yosei] says, it is [derived] from the cities of the Levites. [Note: Per Minchat Bikkurim, Rabbi Yishmael alludes to the proportion of 1 in 30, the approximate ratio of Levites (23,000, see Num. 26:62) to the remaining tribes (601,730, see Num. 26:51) before entering the Land of Israel.]