An ass - The ass could not be offered in sacrifice, being an unclean animal: possibly the only unclean animal domesticated among the Israelites at the time of the Exodus. This principle was extended to every unclean beast; see Numbers 18:15.
Thou shalt redeem - The lamb, or sheep, was given to the priest for the service of the sanctuary.
Firstborn of man - The price of redemption was fixed at five shekels of the sanctuary: Numbers 3:47, where see the note.