Jerusalem Talmud Bava Metzia 7.3

משנה: הָיָה עוֹשֶׂה בַתְּאֵנִים לֹא יֹאכַל בָּעֲנָבִים בָּעֲנָבִים לֹא יֹאכַל בַּתְּאֵנִים אֲבָל מוֹנֵעַ הוּא אֶת עַצְמוֹ עַד שֶׁמַּגִּיעַ לִמְקוֹם הַיָּפוֹת וְאוֹכֵל. וְכוּלָּן לֹא אָמְרוּ אֶלָּא בְשָׁעַת גְּמַר מְלָאכָה אֲבָל מִשּׁוּם הָשֵׁב אֲבֵידָה לַבְּעָלִים אָמְרוּ הַפּוֹעֲלִין אוֹכְלִין בַּהֲלִיכָתָן מֵאֵימֶן לְאֵימֶן וּבַחֲזִירָתָן מִן הַגַּת וַחֲמוֹר שֶׁתְּהֵא פוֹרֶקֶת.

MISHNAH: If one was working on figs he may not eat grapes, on grapes he may not eat figs; but he may restrain himself until he reaches the place of the good ones and eat there. In all cases they only referred to the time of finishing work, but because of returning lost property to its owner they said that workers may eat when they go from row to row, and when they return from the wine press, and a donkey while offloading.

הלכה: הָיָה עוֹשֶׂה בַתְּאֵינִים כול׳. אָמַר רִבִּי לָא. כְּתִיב כִּי תָבֹא בְּכֶרֶם רֵעֶךָ וְאָכַלְתָּ עֲנָבִים כְּנַפְשְׁךָ שָׂבְעֶךָ. וְכִי מַה יֵשׁ בַּכֶּרֶם לוֹכַל אֶלָּא עֲנָבִים. לְלַמְּדָךְ הָיָה עוֹשֶׂה בָּעֲנָבִים לֹא יֹאכַל בַּתְּאֵינִים.

HALAKHAH: “If one was working on figs,” etc. Rebbi La said, it is written: “If you come into your neighbor’s vineyard, you may eat grapes to satisfy yourself.” What else is there in the vineyard to eat but grapes? To teach you that if he was working on grapes he may not eat figs.