Tractate Bekhorot - Chapter 2 - Mishnah 7

If two ewes which had never previously given birth bore two males, both belong to the priest. [If they gave birth] to a male and a female, the male belongs to the priest. [If they gave birth] to two males and a female, 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. 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] two females and a male or two males and two females, the priest receives nothing in such circumstances.