Chabad.org — English translation + Rashi
Torah Temimah points to Horayos 5a (unit 4; single primary locus per emergency Core plan).
E-Daf.com — Horayos 5a
HebrewBooks / Shas PDF
English: If the beit din ruled and one tribe acted on their word — that tribe is liable.
Single-tribe liability — a single tribe that follows an erroneous Sanhedrin ruling brings its own bull.
Liability for the par he’elam davar is not limited to the nation as a whole. When one shevet alone acts upon the Sanhedrin’s mistaken hora’ah, that tribe itself becomes obligated to bring the communal bull. The scope of the offering tracks the scope of the actual transgression.