Halakhah zu Wajikra 10:6

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."

And the prevention was already repeated with the high priest, and it stated (Leviticus 10:6), "and he shall not let his head be wild." However it is repeated in order that we not think that that which is stated to Elazar and Itamar, "you shall not let your head be wild," would only be from the angle of the dead alone; and that when they do this not from the angle of mourning, it would be permissible. Therefore, it is elucidated with the high priest that it is because of the service, that God, may He be blessed, commanded them to cut their hair (see Sefer HaMitzvot LaRambam, Mitzvot Lo Taase 163).

And [it] is practiced at the time of the [Temple] by the males of the priesthood. And one who transgresses it and enters the Temple with wild [hair] - meaning after he grows his hair for thirty days - and serves there is liable for death by the hand of the Heavens; as it is stated, (Leviticus 10:6), "you shall not let your head be wild [...] and you shall not die." And the words of the Torah are [in short] - meaning to say, behold if they let their heads be wild, they will die. Nonetheless their service is not disqualified. And one who entered there but did not serve, is with a warning - meaning to say, he violated a negative commandment and is lashed. But he is only in [the category of] the death penalty if he served - since so did the explanation come. And Ramban, may his memory be blessed, wrote (in Sefer HaMitzvot, Mitzvot Lo Taase 163) that one with wild [hair] not enter the Temple is an embellishment from [the Rabbis] and is not from Torah writ; and his proofs are in his book. And he said that the verse is only coming to forbid drawing close for service, and it is included in the negative commandment of one with a blemish who served. And he wrote further that even the Sages only forbade the place that is called, "between the chamber and the altar"; but they did not forbid in front of the altar itself - which is thirty-two ells, as we learned (Mishnah Middot 3:1, 6), "The altar was thirty-two ells [...]; between the chamber and the altar was twenty-two."