Les Nombres 15:18 Halakha : Mishneh Torah, Heave Offerings, Mishneh Torah, Forbidden Intercourse, Shulchan Arukh, Yoreh De'ah & Sefer HaChinukh

The priestly tithe at this time is not from Torah writ, but [rabbinic] - even in a place that those that came up from Babylonia held, and even at the time of Ezra - as you do not have the priestly tithe from the Torah except in the Land of Israel at the time that all of Israel is there, as it is stated, "When you come" (seemingly a reference to Numbers 15:18, which has, "In your coming") - the coming of all of you, like they were at the first possession, and like they are to return in the future for the third possession, and not like they were at the possession in the days of Ezra, which was a coming of some of them - and hence it was not from the Torah [then]. And so does it appear to me is the law of tithes [as well], that we are only obligated [rabbinically] at this time - like the priestly tithe.

...In the present era, even in Eretz Yisrael,8 challah does not have the status of a Scriptural commandment. [This is derived from Numbers 15:18:] "When you come into the land...."9 [Implied is when] all of you enter and not when only a portion enter.10 When Ezra ascended [to Eretz Yisrael], the entire people did not ascend.Similarly, in the present era, terumah is a Rabbinic commandment.11 Therefore it is eaten by the priests of our era [whose lineage is established merely] by presumption.

We are not obligated in challah from the Torah other than in the land of Israel alone, as it is said: "and it shall be that when you eat from the bread of the land..." (Num. 15:19). And [it applies] at a time that all of Israel is there, as it is said: "when you [plural] come" (Num. 15:18) - the coming of all of you and the coming of some of you. Therefore, [the obligation of ] challah at this time - even in the days of Ezra in the land of Israel - is only from their [i.e. the Sages'] words [i.e. it is of Rabbinic force only].