Halakhah zu Jechezkiel 44:20

That the priests not enter the Temple with grown hair: That the priests not enter the Temple with grown hair, like mourners do - meaning to say that they not grow their hair long, as it is stated (Leviticus 10:6), "you shall not let your head be wild." And the Targum (Aramaic translation of Onkelos) said, "Do not increase locks." And Yechezkel the prophet elucidated and said (Ezekiel 44:20), "and they shall not send forth locks." And so too with the metsora (Leviticus 13:45), "and his head shall be wild" - and they said in Sifra [that it means], "He grows locks."

That He prohibited the priests from entering the Temple with overgrown hair, like that of mourners who do not cut their hair. And that is His saying, "you shall not let your head be wild (tifraau)" (Leviticus 10:6). And Yechezkel explained and said (Ezekiel 44:20), "and they shall not send forth locks (fera)." And likewise did He say with the metzora, (Leviticus 13:45), "and his head shall be wild." And we say in the Sifrei (Sifrei Bamidbar 25:1), "He grows locks."And the language of the Sifra (Sifra Shemini, Mechilta d'Miluim 2:40) is likewise, "'You shall not let your head be wild' - do not grow hair." And this prohibition was already repeated with a high priest, when He said, "and he shall not let his head be wild" (Leviticus 21:10). However it was repeated so that you not think that His saying to Elazar and Itamar, "you shall not let wild," was only for the sake of the dead - but if one did it not in the way of mourning, it is permitted. And hence it comes with the high priest, to [teach] that it is on account of the [Temple] service. And one who transgresses this negative commandment - meaning he serves [in the Temple] with unkempt hair - is [punished] with death. And among those things [listed in Sanhedrin 83a as things punished] by death is unkempt hair. [This is] on account of what He said, "so that you do not die." However if one enters the Temple with unkempt hair but does not serve, behold that it is [only] a prohibition, and not [punished] with death. (See Parashat Shemini; Mishneh Torah, Admission into the Sanctuary 1.)