Tosefta for Bava Metzia 9:5
הַמְקַבֵּל שָׂדֶה מֵחֲבֵרוֹ וְלֹא עָשְׂתָה, אִם יֶשׁ בָּהּ כְּדֵי לְהַעֲמִיד כְּרִי, חַיָּב לְטַפֵּל בָּהּ. אָמַר רַבִּי יְהוּדָה, מַה קִּצְבָּה בַכְּרִי. אֶלָּא אִם יֶשׁ בָּהּ כְּדֵי נְפִילָה:
If one received a field from his neighbor [for half, a third, or a quarter of the yield, as a tenant-farmer], and it did not yield [i.e., it yielded very little; and the tenant-farmer came to him to (inform him that he would) stop working it, it not being worth his effort] — If there is enough in it (the field) to cover the winnowing shovel (leha'amid kri) [close to two sa'ah], he must work it [perforce]. R. Yehudah said: What kind of measurement is kri? [The kri measurement is not appropriate for both a large and a small field, for the cultivation of the one is not comparable to that of the other.] Rather, (the criterion is) if there is enough (to equal) the dropping (of seed). [If the measure of the yield equals that of the sowing, he must cultivate it. The halachah is not in accordance with R. Yehudah.]