Negative commandment · Lo Ta’aseh #144 of 613
Not to eat an unblemished firstling animal outside of Jerusalem
שלא לאכול בכור תמים חוץ לירושלים
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
Not to eat an unblemished firstling animal outside of Jerusalem, as [Deuteronomy, ibid.] states: "You may not eat [within your gates]... and the firstlings [of your cattle and flocks]...."
שלא לאכול בכור תמים חוץ לירושלים, שנאמר "לא תוכל . . . ובכורות" (דברים יב,יז).
Who & when
“And there in the Order of Bo el Pharaoh, I wrote at what time and in what place the commandment of the firstborn is practiced and the disagreement among my teachers — G-d protect them — about the matter of the firstborn at this time.”
Where in the Mishneh Torah
Hilchot Firstlings הלכות בכורות