2. The Mitzvah of Pidyon Haben in Scripture:

Parshas Bo:[1] Hashem spoke to Moshe asking him to command the Jewish people: Every male firstborn of the womb of a person or animal is to be sanctified to me. The firstborn male is to be redeemed. If your children ask what this [Mitzvah] is [about], then you shall tell him that Hashem took us out from slavery with a strong hand, and he killed the Egyptian firstborns, and therefore I sacrifice to G-d my firstborn male animals, and redeem my firstborn sons.

Parshas Ki Sisa:[2] Every firstborn is Mine, the firstborn male of an ox or sheep. The firstborn donkey is to be redeemed with a sheep, or have its neck broken. All the firstborns of your sons should be redeemed.

Parshas Bamidbar: The Levites were taken from amongst the Jewish people in exchange for the firstborns. “The firstborns became Mine on the day I smote the firstborn Egyptians, and I sanctified the firstborn humans and animals to be Mine. The Levites are thus Mine in place of the firstborns.”

Parshas Korach:[3] The firstborn of a person and animal is to go to the Kohanim. The firstborn human and impure animals [i.e. donkey] are to be redeemed. The firstborn son is to be redeemed with five Shekalim after 30 days. The firstborn of pure animals is not to be redeemed, and is rather to have its blood and fat offered to the altar, as it is holy. You are, however, to eat its meat.

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[1] Bo 13:13; Source of Yerushalmi Kiddushin 1:7 for Mitzvah

[2] Ki Sisa 34:19 [Source of Rambam Bikkurim 11:1] Ki Sisa 34:20; Source of Kiddushin 29a

[3] Korach 18:15; Source of Rambam Bikkurim 11:1