Deut 14:21

14:21 a young goat in its mother’s milk. This prohibition is not fully explained. Appearing also in Ex. 23:19 and 34:26, it is the basis of the practice among orthodox Jews of not eating milk and meat products together. The prohibition may be similar to 22:6, which forbids taking the mother bird with its young. There, the idea is to preserve the mother and nest so as to have more birds for the future. Some have also suggested that cooking a goat in milk may have been a Canaanite practice with pagan religious implications, but the evidence is not clear.