The 181st mitzvah is that we are commanded regarding the eglah arufah (calf that is decapitated), in a case where a murder victim is found in a field and we don't know who the murderer was.

The source of this commandment is G‑d's statement1 (exalted be He), "When a corpse is found fallen in the field...." This is the law of eglah arufah.

The details of this mitzvah are explained in the last chapter of tractate Sotah.