Tractate Bekhorot - Chapter 2 - Mishnah 1 בכורות פרק ב׳ משנה א׳

[An Israelite] who buys a fetus of a cow belonging to a non-Jew or who sells one to him, although this is not permitted, or who forms a partnership with him, or who receives [an animal] from him to look after or who gives [his cow] to him to look after, is exempt from the [law of the] bekhor, for it says: [“I sanctified to Me all the firstborn] in Israel,” (Numbers 3:13) but not in non-Jews. Priests and Levites are subject [to the law of the first-born pure animal]. They are not exempt from [the law of] the first-born of a clean animal, but only of a first-born son and the first-born of a donkey.