Reference for Sheviit 3:5

לֹא יִפְתַּח אָדָם מַחְצֵב בַּתְּחִלָּה לְתוֹךְ שָׂדֵהוּ, עַד שֶׁיִּהְיוּ בוֹ שָׁלֹשׁ מוּרְבִּיּוֹת, שֶׁהֵם שָׁלֹשׁ עַל שָׁלֹשׁ עַל רוּם שָׁלֹשׁ, שִׁעוּרָן עֶשְׂרִים וְשֶׁבַע אֲבָנִים:

[During the seventh year], one may not open a quarry within his field, unless there is in it [enough stone to form] three layers, three by three [cubits] by three [cubits] high, [totaling] twenty-seven [one cubit-sized] stones.

Jastrow

Jastrow