Reference for Bava Metzia 9:9
הַמְקַבֵּל שָׂדֶה מֵחֲבֵרוֹ לְשָׁנִים מֻעָטוֹת, לֹא יִזְרָעֶנָּה פִשְׁתָּן, וְאֵין לוֹ בְקוֹרַת שִׁקְמָה. קִבְּלָהּ הֵימֶנּוּ לְשֶׁבַע שָׁנִים, שָׁנָה רִאשׁוֹנָה יִזְרָעֶנָּה פִשְׁתָּן, וְיֶשׁ לוֹ בְקוֹרַת שִׁקְמָה:
If one received a field from his neighbor for few years, [less than seven], he may not sow it with flax, [for flax seed greatly weakens the soil, and its roots remain in the ground for seven years], and he has no (cutting privileges) in the trunk of a sycamore, [a wild fig tree, whose branches are cut for beams.] If he received it from him for seven years, the first year he may sow it with flax, and he has (cutting privileges) in the trunk of a sycamore. [The sycamore is a wild fig tree whose branches are cut for beams. When cut, they grow again, but in fewer than seven years, they cannot be made into beams. Therefore, if he received it for fewer than seven years, he may not cut it for beams, not having received it for this purpose. For in a few years they cannot grow back (to be suitable) for beams. But if he received it for seven years, the first year he may sow it with flax, and he may cut the sycamore that is in it.]