TT #330 · Parashas Shoftim · Seventh Aliyah · Passuk 21:01
If a slain person is found in the land that the Eternal your God is giving you to possess, lying in the field, and it is not known who struck him.
Torah Temimah points to Sotah 44b / Sifrei.
TT lemma: כִּי יִמָּצֵא חָלָל.
English: ‘If a slain person is found’ – and not one who was strangled, and not one who was dismembered.
Derash defining the corpse that requires the rite (Bavli / Sifrei) — the verse implies a body found slain by the sword or similar, not every death.
The eglah arufah is performed only when a slain body is found in the open; certain other forms of death do not trigger the rite.