Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 79
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 79, with English translation by Maimonides (Rambam).
לקדש בכור הבהמה הטהורה ולהקריבו, שנאמר: "כל הבכור אשר יולד" (דברים טו,יט).
To sanctify the firstborn of a kosher animal and offer it as a sacrifice, as [Deuteronomy 15:19] states: "Every firstling which shall be born...."