משנה אויף במדבר 3:45

[An Israelite] who buys a fetus of a donkey 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 donkey] 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 exempt through [an argument made by a] kal vehomer: if they exempted the first-born belonging to the Israelites in the wilderness, it follows all the more so that they should exempt their own.

[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.