Midrash sur Le Lévitique 4:27

1) (Vayikra 4:27): ("And if a single soul sin unwittingly of the people of the land in doing it …"): "soul," "single," "in doing it": These (expressions are redundant [in the Hebrew] and) signal (three) exclusions. (One exclusion:) One who acts on his own is liable (for a sin-offering), and not one who acts on the ruling of beth-din.

1 (Vayikra 4:28): "If his sin becomes known to him (wherein he sinned"): And not if others tell him. I might think even if he does not refute (them); it is, therefore, written (to negate this): "if it becomes known."

2) How so? If beth-din (mistakenly) ruled (that it is permitted) to transgress one of all the mitzvoth mentioned in the Torah, and an individual went and acted on their ruling — whether they themselves acted (on their ruling) and he acted with them, whether they acted and he acted after them, whether they did not act and he acted — I might think he is liable; it is, therefore, written: "soul," "single," "in doing it" — these are exclusions, (one of which relates to such an instance.) One who acts on his own is liable, and not one who acts on the ruling of beth-din.