וְכָל־בְּהֵמָ֞ה מַפְרֶ֣סֶת פַּרְסָ֗ה וְשֹׁסַ֤עַת שֶׁ֙סַע֙ שְׁתֵּ֣י פְרָס֔וֹת מַעֲלַ֥ת גֵּרָ֖ה בַּבְּהֵמָ֑ה אֹתָ֖הּ תֹּאכֵֽלוּ׃
Y todo animal de pezuñas, que tiene hendidura de dos uñas, y que rumiare entre los animales, ese comeréis.
Mishnah Chullin
If an animal was having difficulty giving birth and the fetus put forth a limb and then put it back in, it may be eaten [when its mother is slaughtered]. If it put forth its head, even though it put it back in, it is considered as born. Whatever is cut off from the fetus within the womb [and left inside] may be eaten, but whatever is cut off from the spleen or kidneys [of the animal and left inside] may not be eaten. This is the rule: that which is from the body of the animal is forbidden, but that which is not from the body of the animal is permitted.