Mishnayos Nedarim Perek 7 Mishnah 2

For one who vows that grain [dagan] is forbidden to him, it is prohibited to eat the dry cowpea, because, like grain, its final stage of production involves being placed in a pile; this is the statement of Rabbi Meir. And the Rabbis say: It is prohibited for him to partake of only the five species of grain: Wheat, barley, oats, spelt, and rye, as that is the connotation of the term dagan in the Torah. Rabbi Meir says: For one who vows that grain is forbidden to him, and therefore he will refrain from eating grain [tevua], it is prohibited for him to eat from only the five species of grain. However, for one who vows that grain is forbidden to him, and therefore he will refrain from eating grain [dagan], it is prohibited to eat all produce whose final stage of production involves being placed in a pile, e.g., dry cowpea, and it is permitted for him to eat fruits of the tree and vegetables.

הַנּוֹדֵר מִן הַדָּגָן, אָסוּר בְּפוֹל הַמִּצְרִי יָבֵשׁ, דִּבְרֵי רַבִּי מֵאִיר. וַחֲכָמִים אוֹמְרִים, אֵינוֹ אָסוּר אֶלָּא בַחֲמֵשֶׁת הַמִּינִין. רַבִּי מֵאִיר אוֹמֵר, הַנּוֹדֵר מִן הַתְּבוּאָה, אֵינוֹ אָסוּר אֶלָּא מֵחֲמֵשֶׁת הַמִּינִין. אֲבָל הַנּוֹדֵר מִן הַדָּגָן, אָסוּר בַּכֹּל, וּמֻתָּר בְּפֵרוֹת הָאִילָן וּבְיָרָק: