Positive commandment · Mitzvat Aseh #96 of 613
For everyone who comes in contact with the corpse of an animal to become impure
להיות כל נוגע בנבילה טמא
The verse · Vayikra 11:39
וְכִי יָמוּת מִן־הַבְּהֵמָה אֲשֶׁר־הִיא לָכֶם לְאׇכְלָה הַנֹּגֵעַ בְּנִבְלָתָהּ יִטְמָא עַד־הָעָרֶב
“And if any beast, of which you may eat, die; he that touches its carcass shall be unclean until evening.”
The Rambam’s words
For everyone who comes in contact with the corpse of an animal to become impure, as [Leviticus 11:39] states: "Should an animal die...."
להיות כל נוגע בנבילה טמא, שנאמר "וכי ימות מן הבהמה" (ויקרא יא, לט).
Why · Sefer HaChinuch #161
Matter of the impurity of a carcass
“From the roots of the commandment, [we need to] preface [that] anyone who has intelligence will not have a doubt that there was never a man that grasped to know all wisdom to its end, that nothing of it be hidden from him.”
Where in the Mishneh Torah
Hilchot Other Sources of Defilement הלכות שאר אבות הטומאות