The Torah commands לא תבשר גדי בחלב אמו three times (Shmos 28,19; 34,26; Devarim 14,21). Thus, the Torah is clear that this unique prohibition is not to cook meat with milk (two permitted items become prohibited when cooked together).
From where does Chazal understand the source for the additional prohibition of eating and of benefitting from “meat with milk”? (First six reasons brought in Chullin 114-116)
Rav Ashi
לא תאכל כל תועבה (Devarim 14,3) “You should not eat any abomination”
This verse teaches that anything that is abominated by the Torah, such as “meat with milk” is prohibited. This applies to benefits as well when prohibited not to eat.
Reish Lakish
אל תאכל נא ובשל מבשל (Shmos 12,9) “Do not eat it undercooked or boiled in water”
The extra mention of the term mevashal, written regarding the Korban Pesach, teaches that “meat with milk” is prohibited for consumption and benefit, similar to a cooked Korban Pesach (Kodshim are prohibited with benefit).
Rebbi
לא תאבלנו (Devarim 12,25) “Do not eat it”
Extra phrase stated in reference to the blood of disqualified offerings. Common term קדש shared by “meat with milk” and promiscuity source for prohibition for benefits.
Academy of Rabbi Eliezer
לא תאכלו כל נבלה (Devarim 14,21) “You should not eat any nevilah”
Teaches that even when you sell a nevilah to the gentile, you are not permitted to cook it with milk and then sell it.
Academy of Rabbi Yishmael
לא תבשר גדי בחלב אמו three times (Shmos 28,19; 34,26; Devarim 14,21) “Do not cook a kid in the milk of its mother”
Torah states the prohibition three times to teach prohibited for “meat with milk” to be eaten, to derive benefit or to cook.
Issi ben Yehudah
Prohibition to consume derived from common term קודש stated in reference to “meat with milk” (Devarim 14,21) and in reference to terifah (Shmos 22,30). Prohibition from benefit is derived from join derivation from orlah, chametz, and kilayim.
Rabbi Shimon ben Yehudah
Prohibition to consume derived from common term קודש stated in reference to “meat with milk” (Devarim 14,21) and in reference to terifah (Shmos 22,30). However, just as benefit is permitted by terifah so too regarding “meat with milk”.
Pesikta Zutarta
לא תאכל הנפש עם הבשר (Devarim 12,23) “Do not eat the soul (blood) with the flesh”
Derive prohibition to eat “meat with milk” since milk is a derivative of blood.
Rambam
Since the Torah prohibits cooking meat with milk, it is understood that eating “meat with milk” is also prohibited (similar to the understanding of that the prohibition of incest with one’s daughter is not stated but is assumed since the Torah prohibits one’s granddaughter.
Not only eating & benefiting, but also cooking (preparation) is forbidden in mixing of milk & meat; concept to avoid cruelty. The lesson to learn is that we must take extra care to refrain from being insensitive to another person. (Lekutei Sichos Volume 6)
