Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 96
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 96, with English translation by Maimonides (Rambam).
להיות כל נוגע בנבילה טמא, שנאמר "וכי ימות מן הבהמה" (ויקרא יא, לט).
For everyone who comes in contact with the corpse of an animal to become impure, as [Leviticus 11:39] states: "Should an animal die...."