פירוש על עבודה זרה 58:6

Rabbi Ishmael responds that a priest who sees rennet in the stomach of a whole burnt offering may drink this rennet (I know this sounds a little gross.). Since a whole burnt offering is forbidden to be eaten, this law proves that the rennet is not considered part of the animal. If it were considered to be an integral part of the animal, it would have been prohibited. Since it is not forbidden in this case then by comparison the rennet used to make cheese should not make the cheese forbidden to Jews, even though it comes from an animal that was improperly slaughtered.
At this point the mishnah offers an aside. The statement that the rennet of a whole burnt offering may be consumed by a priest was only Rabbi Ishmael’s opinion. The Rabbis did not agree with him. They stated that it is forbidden to derive benefit from this rennet and yet one who does so is not considered to have taken (trespassed) Temple property. [If he had taken from Temple property he would have to bring a sacrifice to atone for the sin, as well as pay back the value of that which he took.] This section is not an integral part of the mishnah and was added in at a later time.