Mishnayos Moed Katan Perek 2 Mishnah 3

A person may bring his fruit in from the field on the intermediate days of the Festival because he is concerned about thieves, and he may draw his flax out of the soaking pool so that it is not ruined from soaking too long in the water, provided that he does not plan from the outset to perform his work on the intermediate days of the Festival. And with regard to all of these cases, if one planned from the outset to perform his labor on the intermediate days of the Festival, the fruit of that labor must be lost and no benefit may be derived from it.

מַכְנִיס אָדָם פֵּרוֹתָיו מִפְּנֵי הַגַּנָּבִים, וְשׁוֹלֶה פִשְׁתָּנוֹ מִן הַמִּשְׁרָה בִּשְׁבִיל שֶׁלֹּא תֹאבַד, וּבִלְבַד שֶׁלֹּא יְכַוֵּן אֶת מְלַאכְתּוֹ בַמּוֹעֵד. וְכֻלָּן אִם כִּוְּנוּ מְלַאכְתָּן בַּמּוֹעֵד, יֹאבֵדוּ: