Tractate Bekhorot - Chapter 2 - Mishnah 8

If one [of the ewes] had given birth and the other had never previously given birth and they bore two males, one remains with him and the other belongs to the priest. Rabbi Tarfon says: the priest chooses the better one. Rabbi Akiva says: we compromise between them. The second one [in the Israelite's possession] is left to pasture until it becomes blemished and the owner is liable for the [priest's] gifts. Rabbi Yose exempts him. For Rabbi Yose says: wherever the priest receives [an animal] in its place, he is exempt from the priestly gifts. Rabbi Meir however makes him liable. If one of them died: Rabbi Tarfon says: they divide [the living one]. Rabbi Akiva says: the burden of proof is upon the claimant. If they gave birth to a male and a female, the priest receives nothing [in such circumstances].