Mishnayos Chullin Perek 8 Mishnah 5

The congealed milk in the stomach of the animal of a gentile and of an unslaughtered animal carcass is prohibited. With regard to one who curdled milk by using the skin of the stomach of a kosher animal as a coagulant to make cheese, which may then have the taste of meat cooked in milk, if the measure of the skin is enough to impart flavor to the milk, that cheese is prohibited. In the case of a kosher animal that suckled milk from a tereifa, the milk in its stomach is prohibited, as the milk is from the tereifa. If it was a tereifa that suckled milk from a kosher animal, the milk in its stomach is permitted, as the milk is from the kosher animal. In both cases, the milk that an animal suckles has the status of the animal from which it was suckled, and not that of the animal which suckled, because the milk is collected in its innards and is not an integral part of its body.