Midrasz do Kapłańska 25:3

2) (Vayikra 25:2) ("Speak to the children of Israel and say to them: When you come to the land that I give to you, the land shall rest a Sabbath to the L–rd.") "When you come": I might think (that this phrase subsumes) coming to Trans-Jordan; it is, therefore, written "to the land" — the specific land (i.e., Eretz Yisrael). I might think that it subsumes) coming to Ammon and Moav; it is, therefore, written "that I give to you," and not to Ammon and Moav. Whence do we derive that if they conquered (the land) but did not yet divide it (among themselves), or if they divided it by families, but not by fathers' houses, and no one recognizes his (specific) portion — I might think that shemitah is (still) binding on them. It is, therefore, written (Vayikra 25:3) "your field" — each one must recognize his field; "your vineyard" — each one must recognize his vineyard.