TT #157 · Parshas Shemini · Aliyah 6 · Passuk 27
Chabad.org — Vayikra Chapter 11 with Rashi
English: And whatever walks on its paws among all the animals that walk on four legs shall be impure for you; whoever touches their carcass shall be impure until evening.
Sifra (Toras Kohanim), Shemini, Chapter 6.
Sefaria — Sifra, Shemini, Chapter 6
הַהֹלֶכֶת עַל אַרְבַּע. לְרַבּוֹת חַיּוֹת טְמֵאוֹת בְּכָל הַחַיָּה, לְרַבּוֹת אֶת הַפִּיל.
Comprehensive extension — The general wording includes other impure land animals and the elephant within the category.
The wording extends the category to impure animals and specifically includes the elephant.