Positive commandment · Mitzvat Aseh #112 of 613
For a person afflicted with tzara’at to make known his condition to all others
להיות המצורע ידוע לכל בדברים האמורים בו "בְּגָדָיו יִהְיוּ פְרֻמִים וְרֹאשׁוֹ יִהְיֶה פָרֻעַ וְעַל שָׂפָם יַעְטֶה וְטָמֵא טָמֵא יִקְרָא" . וכן כל שאר הטמאים צריכין להודיע את עצמן
The verse · Vayikra 13:45
וְהַצָּרוּעַ אֲשֶׁר־בּוֹ הַנֶּגַע בְּגָדָיו יִהְיוּ פְרֻמִים וְרֹאשׁוֹ יִהְיֶה פָרוּעַ וְעַל־שָׂפָם יַעְטֶה וְטָמֵא טָמֵא יִקְרָא
“And the diseased man in whom the plague is, his clothes shall be rent, and the hair of his head shall grow long, and he shall put a covering upon his upper lip, and shall cry, Unclean, unclean.”
The Rambam’s words
For a person afflicted with tzara’at to make known his condition to all others, according to the instructions mentioned in [Leviticus 13:45]: "His garments shall be torn, his hair shall grow uncut, he shall cover his face to the lip, and he shall cry out: `Impure! Impure!"' Similarly, all others who are ritually impure must make known their condition.
להיות המצורע ידוע לכל בדברים האמורים בו "בְּגָדָיו יִהְיוּ פְרֻמִים וְרֹאשׁוֹ יִהְיֶה פָרֻעַ וְעַל שָׂפָם יַעְטֶה וְטָמֵא טָמֵא יִקְרָא" (ויקרא יג, מה). וכן כל שאר הטמאים צריכין להודיע את עצמן.
Why · Sefer HaChinuch #171
The practice of wildness and rending of a metzora and all who render a person impure
“It is from the root of the commandment that he take his distancing from people as an example for his soul. As a person is distanced from all good as a result of sin, in order that he will repent from his evil way.”
Who & when
“And [it] is practiced at all times that we have priests and sages that know the ailments, that are fitting to render pure and to render impure.”
Where in the Mishneh Torah
Hilchot Defilement by Leprosy הלכות טומאת צרעת