Negative commandment · Lo Ta’aseh #145 of 613
For the priests not to eat a sin offering or a guilt offering outside the Temple courtyard
שלא יאכלו הכוהנים חטאת ואשם חוץ לעזרה
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 the priests not to eat a sin offering or a guilt offering outside the Temple courtyard, as [Deuteronomy, ibid.] continues: "...your cattle and your flocks." According to the oral tradition, the purpose of this phrase is only to forbid the consumption of the sin offerings and the guilt offerings outside the Temple courtyard. Anything that is eaten outside the place intended for it is covered by the [prohibition]: "You may not eat within your gates...."
שלא יאכלו הכוהנים חטאת ואשם חוץ לעזרה, שנאמר "לא תוכל . . . בקרך, וצאנך" (דברים יב,יז); מפי השמועה למדו שלא בא הכתוב אלא לאסור בשר חטאות ואשמות חוץ לעזרה, לפי שכל דבר שייאכל חוץ למקום אכילתו "לא תוכל לאכול בשעריך" (שם) אקרא בו.
Why · Sefer HaChinuch #446
To not eat higher-level consecrated foods (kodshei kodashim) outside of the Temple yard
“It is from the roots of the commandment to eat every sacrifice in its place so that its eaters will focus their hearts on the atonement for which they are eating.”
Who & when
“And this prohibition is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Sacrificial Procedure הלכות מעשה הקרבנות