Negative commandment · Lo Ta’aseh #153 of 613
To not eat tevel
שלא לאכול הטבל; והטבל הוא הדבר שגידוליו מן הארץ החייב בתרומות ומעשרות, שלא הוציאו ממנו תרומת ה'
The verse · Vayikra 22:15
וְלֹא יְחַלְּלוּ אֶת־קׇדְשֵׁי בְּנֵי יִשְׂרָאֵל אֵת אֲשֶׁר־יָרִימוּ לַה׳
“And they shall not profane the holy things of the children of Yisra᾽el, which they offer to the Lord;”
The Rambam’s words
Not to eat tevel. Tevel refers to produce from which one is obligated to separate terumah and tithes from which G-d's terumah has not been separated, as [Leviticus 22:15] states: "And they shall not profane the sacred gifts which the children of Israel will separate for G-d." This implies that the produce which [the Jews] will ultimately separate for G-d should not be treated in a mundane manner and eaten while tevel.
שלא לאכול הטבל; והטבל הוא הדבר שגידוליו מן הארץ החייב בתרומות ומעשרות, שלא הוציאו ממנו תרומת ה': .שנאמר "ולא יחללו, את קודשי בני ישראל--את אשר ירימו, לה'" (ויקרא כב,טו); כלומר, דברים שהם עתידין להרים אותם לה', לא יעשו אותם חול ויאכלו אותם בטבלם.
Why · Sefer HaChinuch #284
To not eat tevel
“It is from the roots of the commandment [that it is] to aggrandize and make dear all that is [part of] the holy in the heart of every man. And I have already mentioned many times the benefit that comes out to us in the thing.”
Who & when
“And from that which it appears, the Sages, may their memory be blessed, were stringent (Chullin 6b) in the places about which they were in doubt if they were from the Land of Israel, even about the fruits the tithing obligation of which was only rabbinic — even after the destruction of the Temple, even though this prohibition is only practiced [altogether] from Torah writ [when the Temple is standing].”
Where in the Mishneh Torah
Hilchot Tithes הלכות מעשרות