The 163rd prohibition is that kohanim are forbidden from entering the Temple grounds with disheveled [long] hair like those in mourning, who do not set or arrange their hair.

The source of this prohibition is G‑d's statement1 (exalted be He) [to Aaron and his sons,] "Your heads al tif'rau." The Targum translates, "Do not grow your hair." In Ezekiel,2 this is further explained — "Do not allow your hair to grow long."

So too, G‑d's statement3 (exalted be He), regarding the leper, "His head shall be farua," is explained in Sifra, "he must grow his hair."4

The Sifra explains [our mitzvah in the same way], "The phrase, 'Your heads al tif'rau,' refers to not growing it long."

This prohibition is repeated regarding the kohen gadol in the verse,5 "He may not allow his hair to grow long." The reason for this repetition is so one would not think that the commandment to Elazar and Isamar,6 "Do not grow your hair long" ("Your heads al tif'rau") was only because of the death [of their brothers Nadav and Avihu, and that growing their hair was prohibited because otherwise people would think they were mourning]; but if one did so without mourning, then it is allowed. Therefore it explains regarding the kohen gadol that his hair must be cut for the Temple service [even when he is not mourning].

One who transgresses this prohibition and performs the service with long hair is punished by a [Divine] death sentence. Those [who perform the service] with long hair are listed7 among, "these that are punished by death," the source being the verse,8 "otherwise you will die."

If, however, one enters the Temple grounds with long hair without performing any service, he transgresses this prohibition but is not punished by death.