Commentary for Exodus 34:19

כל פטר רחם לי EVERY FIRST OFFSPRING OF THE WOMB IS MINE amongst human beings,

כל פטר רחם לי, the commandment applies to male humans and male new born animals of the categories that are domesticated and fit to eat for Jews. Seeing that one category of “unclean” animal, the donkey, is also part of this legislation, though, of course such an animal is not fit to be sacrificed on the altar of the Temple, the Torah has to add certain qualifications when writing about this commandment. Whereas the “pure” firstborn may not be redeemed and made secular, but must themselves be offered on the altar, their blood and fat remaining there, the donkey either has to be redeemed or to be killed in a manner that precludes its flesh being used commercially. [it may not be consumed seeing it is an “unclean” animal. Ed.] (Numbers 18,17)

כל פטר רחם לי, seeing that through the plague of the killing of the firstborn of Egypt all the Jewish firstborn sons and firstborn male livestock who left Egypt were sanctified. The legislation was recorded after the legislation dealing with festival of matzot.