Sefer HaChinuch ספר החינוך

That an impure priest not serve: That a priest not serve when he is still impure, as it is stated (Leviticus 22:2), “and they shall separate from the holy things of the Children of Israel and they shall not profane My holy name.” And they, may their memory be blessed, said in the ninth chapter of Sanhedrin 83b, “From where [do we know] about an impure one who served that he is [punished] with death” — meaning to say, with death by the hands of the Heavens? “As it is written, ‘Speak to Aharon and to his sons, and they shall separate from the holy things of the Children of Israel and they shall not profane’”; and it is written in another place (Leviticus 22:9), “and die for it, since they profaned it.”

That which we have written in many places is from the roots of the commandment — [that] because of the glory of the [Temple] and the loftiness of the service, we distance from it anything that is not in its loftiness and its importance — and the loftiness of a man in purity is known to all who understand.

The laws of the commandment are for example, what are the impurities that render impure rabbinically; in what ways one impure purifies himself from his impurity: which impurity requires sprinkling and a sacrifice for its purification and which only needs immersion and the setting of the sun — and included in the matter is that it is impossible to emerge from any impurity [without] immersion; which impurity requires seven days for its purification and which suffices with one day. And the rest of its many matters which “became very numerous,” are all elucidated in the Order of Tahorot. (See Mishneh Torah, Laws of Admission into the Sanctuary 4.)

And [it] is practiced by priests at the time of the [Temple]. And a priest that transgresses it and serves in impurity is liable for death by the hands of the Heavens. (See Mishneh Torah, Laws of Admission into the Sanctuary 4:1.)