The 24th mitzvah is that the kohanim are commanded to wash their hands and feet whenever they need to enter the heichal [main Temple building] or to perform the Temple service. This mitzvah is known as kiddush yadayim v'raglayim.

The source of this commandment is G‑d's statement1 (exalted be He), "Aaron and his sons must wash their hands and feet from it [the washstand] before entering the ohel moed."

One who violates this positive commandment — i.e. a kohen who serves in the Temple without washing his hands and feet — is punished by misah biy'dei shamayim. This is derived from G‑d's statement2 (exalted be He), "They must wash with water if they are not to die."

All the details of this mitzvah are explained in the second chapter of Zevachim.3