Numbers 18:17 "But the first-born of an ox, or the first-born of a sheep, or the first-born of a goat you shall not redeem; they are holy"

At the time of the Exodus from Egypt, HaShem declared that all first-born male children and animals will be sacred to Him.

First-born male children may be "redeemed" in a special procedure called "Pidyon HaBen" (see Positive Mitzvot 79-80).

However, first-born oxen, sheep or goats are considered holy and must be offered as sacrifices. We are forbidden to redeem these first-born animals.