ESV - 19 The best of the firstfruits of your ground you shall bring into the house of the Lord your God. "You shall not boil a young goat in its mother's milk.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
According to Exodus, "It was much used for food (Gen 27:9; 38:17; Judg 6:19; 14:6). The Mosaic law forbade to dress a kid in the milk of its dam, a law which is thrice repeated (Ex 23:19; 34:26; Deut 14:21). Among the various reasons assigned for this law, that appears to be the most satisfactory which regards it as "a protest against cruelty and outraging the order of nature." A kid cooked in its mother's milk is "a gross, unwholesome dish, and calculated to kindle animal and ferocious passions, and on this account Moses may have forbidden it" (Exodus, 23:19).