And the two kidneys and the fat that is on them, that is on the loins; and the lobe on the liver, which he shall remove with the kidneys. (Same formula at 3:10 for sheep and 3:15 for goat.)
Source
Sifra, Vayikra Dibbura deNedavah, on “the two kidneys” (parallel at cattle 3:4 and flock).
English: “And the two kidneys” — not an animal with one kidney and not an animal with three (that is a blemish). “And the fat that is upon them” — and not the flesh that is upon them (which may be eaten). “Which is on the loins” — this is the fat between the loin-sinews, the words of R’ Yossi Haglili; R’ Akiva says: to include the fat on the coccyx.
In Sifra: The thrice-repeated kidney formula (cattle, sheep, goat) fixes the count of two, excludes kidney-flesh from the altar list, and locates the flank-fat either between the loin sinews or at the tail-bone, depending on the Tanna.
Type of Learning
Custom (exact count + exclusion of flesh + definition of kesalim).
What Is Learned
Shelamim require exactly two kidneys; the fat on them goes up, the flesh does not; “on the loins” names a specific strip of fat (sinew-gap or coccyx).