Negative commandment · Lo Ta’aseh #187 of 613
Not to eat meat and milk [together]
שלא לאכול בשר בחלב
The verse · Shemot 34:26
רֵאשִׁית בִּכּוּרֵי אַדְמָתְךָ תָּבִיא בֵּית ה׳ אֱלֹהֶיךָ לֹא־תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ
“The first of the firstfruits of thy land thou shalt bring to the house of the Lord thy God. Thou shalt not boil a kid in its mother’s milk.”
The Rambam’s words
Not to eat meat and milk [together], as [Exodus 34:26] states: "Do not cook a kid in its mother's milk." Based on the oral tradition, we have learned that one [of these verses] implies a prohibition against cooking [the two together], and the other, a prohibition against eating [from the combination].
שלא לאכול בשר בחלב, שנאמר "לא תבשל גדי, בחלב אימו" פעם שנייה (שמות לד,כו); כך למדו מפי השמועה, שאחד לאיסור בישול ואחד לאיסור אכילה.
Why · Sefer HaChinuch #113
To not eat meat with milk
“It is from the roots of this commandment, according to what appears, that it is similar to the matter that we wrote about the commandment of the witch (Sefer HaChinukh 62), that there are things in the world the mixture of which are forbidden to us, for the reason of the matter that we said there.”
Who & when
“And [it] is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Forbidden Foods הלכות מאכלות אסורות