Negative commandment · Lo Ta’aseh #149 of 613
For a priest not to partake of the first fruits [Bikkurim] before they are placed down in the Temple courtyard
שלא יאכל כוהן ביכורים קודם הנחה בעזרה
The verse · Devarim 12:17
לֹא־תוּכַל לֶאֱכֹל בִּשְׁעָרֶיךָ מַעְשַׂר דְּגָנְךָ וְתִירֹשְׁךָ וְיִצְהָרֶךָ וּבְכֹרֹת בְּקָרְךָ וְצֹאנֶךָ וְכׇל־נְדָרֶיךָ אֲשֶׁר תִּדֹּר וְנִדְבֹתֶיךָ וּתְרוּמַת יָדֶךָ
“Thou mayst not eat within thy gates the tithe of thy corn, or of thy wine, or of thy oil, or the firstlings of thy herds or of thy flock, or any of thy vows which thou vowest, or thy freewill offerings, or offering of thy hand:”
The Rambam’s words
For a priest not to partake of the first fruits [Bikkurim] before they are placed down in the Temple courtyard, as [Deuteronomy, ibid.] states: "You may not [eat]... the elevated gifts [delivered by] hand." The [latter phrase] refers to the first fruits.
שלא יאכל כוהן ביכורים קודם הנחה בעזרה, שנאמר "לא תוכל . . . ותרומת ידך" (דברים יב,יז); אלו הביכורים.
Who & when
“And this prohibition is practiced by males and females, but only at the time of the Temple (Mishneh Torah, Laws of First Fruits and other Gifts to Priests Outside the Sanctuary 2:5) — since the obligation to bring first-fruits is then.”
Where in the Mishneh Torah
Hilchot First Fruits and other Gifts to Priests Outside the Sanctuary הלכות ביכורים ושאר מתנות כהונה שבגבולין