Rashi on Exodus 13:13/רש"י על שמות י״ג:י״ג

1פטר חמור AND EVERY FIRST OFFSPRING OF AN ASS — but not the first offspring of other unclean animals. This exception is the enactment of Scripture, because the firstborn of the Egyptians are compared to asses (Ezekiel 23:20; Bereishit Rabbah 96); and a further reason for this exception is because they (the asses) were of assistance to the Israelites when they left Egypt, for there was not a single Israelite who did not take with him from Egypt several asses laden with the silver and gold of Egypt. (Bekhorot 5b).

2תפדה בשה THOU SHALT REDEEM WITH A LAMB — one gives a lamb to the priest, and the first offspring of the ass is then permitted to be made use of, whilst the lamb remains in the priest’s possession with the character of a non-holy object (חולין — an ordinary animal) (cf. Bekhorot 9b).

3וערפתו THEN THOU SHALT BREAK ITS NECK — one breaks its neck with a hatchet from behind and so slays it (Bekhorot 13a). The reason is: he (the owner of the ass) has caused a loss to the possessions of the priest (by not giving him the lamb), therefore must he suffer loss in his own possessions (Mekhilta d['Rabbi Yishmael 13:13:2).

4וכל בכור בניך תפדה AND ALL THE FIRSTBORN OF MAN AMONGST THY SONS SHALT THOU REDEEM — His redemption money is fixed in another passage (Numbers 18:16) at five Sela’im.