Chabad.org — English translation + Rashi
Torah Temimah points to Chullin 93a.
E-Daf.com — Chullin 93a
HebrewBooks / Shas PDF
English: Rav Yehudah said in the name of Shmuel: The fat of the flanks is forbidden and punished with kares — and that is “the fat that is on the loins.”
Identification of flank fat — chelev on the loins is the forbidden fat of the flanks.
Chelev on the loins is the forbidden fat of the flanks. Shmuel’s statement both prohibits that fat under penalty of kares and equates it with the phrase “the fat that is on the loins” in the verse. The ruling supplies a practical anatomical identification of the chelev that the Torah requires to be removed and offered with the kidneys.