Mishnayos Bava Metzia Perek 7 Mishnah 4

If a laborer was performing labor with figs he may not eat grapes; if he was performing labor with grapes he may not eat figs, as he may eat only the type of food with which he is working. This is the halakha even if he was employed to perform labor with both types of produce but is currently performing labor with only one of them. But he may hold himself back from eating until he reaches a place of good-quality grapes or figs and eat from these, as they are the same type of food. And with regard to all of these cases the Sages said that he may eat only at the time of work. But due to the obligation to restore lost property to its owners, i.e., so that workers would not neglect their task, they said that laborers may eat as they walk from one row of a vineyard or plantation to another row, and upon their return from the winepress. And with regard to a donkey, it is permitted to eat when it is being unloaded. This statement will be explained in the Gemara.

הָיָה עוֹשֶׂה בִתְאֵנִים, לֹא יֹאכַל בַּעֲנָבִים, בַּעֲנָבִים, לֹא יֹאכַל בִּתְאֵנִים. אֲבָל מוֹנֵעַ אֶת עַצְמוֹ עַד שֶׁמַּגִּיעַ לִמְקוֹם הַיָּפוֹת וְאוֹכֵל. וְכֻלָּן לֹא אָמְרוּ אֶלָּא בִשְׁעַת מְלָאכָה, אֲבָל מִשּׁוּם הָשֵׁב אֲבֵדָה לַבְּעָלִים אָמְרוּ, פּוֹעֲלִים אוֹכְלִין בַּהֲלִיכָתָן מֵאֻמָּן לְאֻמָּן, וּבַחֲזִירָתָן מִן הַגַּת, וּבַחֲמוֹר כְּשֶׁהִיא פוֹרָקֶת: