Mishnayos Temurah Perek 5 Mishnah 2

ילדה שני זכרים – we are speaking about an unconsecrated animal, if the animal was dedicated, how was something that was sanctified as a burnt-offering let it be a burnt-offering, for the other one refers to the sanctity of its mother. But it is certainly an unconsecrated animal, and therefore, this one whose monetary value was not sanctified is unconsecrated. But however, on both of them, the holiness of a burnt-offering exists. For since he stated that if [he born animal] is a male, it is a burnt-offering, and because he did not vow other than for one, therefore, he should offer the one for his vow, and the second he should sell for the needs of a burnt-offering and its monetary value/worth is unconsecrated.

אין קדושה חלה עליהן – for the offspring become holy because they are holy [at the time of their existence] and not from the belly of their mother, and since that these when they leave [their mother’s womb] are not appropriate for sacrifice, they are not holy. But the Sages dispute Rabban Shimon ben Gamaliel as they hold that the offspring are holy and exist through the holiness of their mother [from the womb]. And the Halakha is not according to Rabban Shimon ben Gamaliel (see Talmud Temurah 25a).