Leviticus 21:1

אמר אל הכהנים SAY UNTO THE PRIESTS [… AND THOU SHALT SAY UNTO THEM] — “Say" and again “thou shalt say unto them" — this repetition is intended to admonish the adults about their children also — that they should teach them to avoid defilement (Yevamot 114a). בני אהרן THE SONS OF AARON — One might think that חללים (priests who have lost their priestly status for reasons connected with their birth or marriage) also may not defile themselves by the dead, Scripture therefore states, "Say unto the priests", — thus only those sons of Aaron are included who have not lost their priestly character; consequently חללים are excluded (Sifra, Emor, Section 1 1) . בני אהרן THE SONS OF AARON — This implies also those of Aaron's sons who have a bodily blemish; בני אהרן THE SONS OF AARON — but not the daughters of Aaron (Sifra, Emor, Section 1 1). לא יטמא בעמיו THERE SHALL NONE BE DEFILED BY THE DEAD AMONG HIS PEOPLES — This means, as long as the dead is among his peoples (i. e. so long as there are some of his people — Jews — who can occupy themselves with his burial) thus excluding the case of a מת מצוה (a corpse of a person whose relatives are unknown or which lies in a place where there are no Jews, nor are there any in the near vicinity; cf. Nazir 43b) in which case the priest is allowed to make himself unclean by handling the corpse (Sifra, Emor, Section 1 3).

וַיֹּ֤אמֶר יְהֹוָה֙ אֶל־מֹשֶׁ֔ה אֱמֹ֥ר אֶל־הַכֹּהֲנִ֖ים בְּנֵ֣י אַהֲרֹ֑ן וְאָמַרְתָּ֣ אֲלֵהֶ֔ם לְנֶ֥פֶשׁ לֹֽא־יִטַּמָּ֖א בְּעַמָּֽיו׃

And the Lord said to Moshe, Speak to the priests the sons of Aharon, and say to them, There shall none be defiled for the dead among his people: