Numbers 31:19

מחוץ למחנה [AND ABIDE YE] OUTSIDE THE CAMP — This means that they should not enter the forecourt of the Sanctuary (the מחנה שכינה cf. Rashi on 5:2). כל הרג נפש WHOSOEVER HATH KILLED A PERSON — Rabbi Meir said: Scripture is speaking of one who killed a person with a weapon receptive of uncleanness, and Scripture teaches you that “a vessel“ (any object) makes a man unclean through the connection thus formed with the corpse just as though he were actually in contact with the corpse itself. Or perhaps one might think that even though he had shot an arrow and killed him (in which case he was not “connected” by means of the arrow with the man he killed) he was to remain outside the camp as being unclean. It, however, says immediately afterwards “and whosoever hath touched anyone slain”; it compares the person who kills to one who touches a slain person. How is it in the case of one who touches a corpse? He becomes unclean in consequence of the connection with it! So, too, one who kills becomes unclean only through some connection with it (the corpse) (Sifrei Bamidbar 127:1 on Numbers 19:16). תתחטאו PURIFY YOURSELVES by means of the waters of sprinkling, as is the law in the case of other people who have become unclean by reason of a corpse. For even according to the opinion of those who say that the graves of heathens do not render Israelites unclean if they are under the same tent (roof) with them (i.e. the graves), because it is said, (Ezekiel 34:31) “And ye, My sheep, the sheep of My pasture, are men”, which is explained to mean: You alone come under the term “men” (Yevamot 61a) — even such will admit that the corpses of heathens render an Israelite unclean through contact with and through carrying them, because the term אדם, man, is only used in reference to uncleanness caused by a “tent”, since it states, (Numbers 19:14) “If a man (אדם) die in a tent". אתם ושביכם [PURIFY] BOTH YOURSELVES AND YOUR CAPTIVES — Not that heathens are receptive of uncleanness and therefore require sprinkling, but such captives are intended as are of the same category as yourselves. What is the case with you yourselves? You are people in covenant with God! So, too, your captives when they come into the covenant with God, if they become unclean they require sprinkling (Sifrei Bamidbar 157:7).

And abide outside the camp seven days: whoever has killed any person, and whoever has touched any slain, purify both yourselves and your captives on the third day, and on the seventh day.

וְאַתֶּ֗ם חֲנ֛וּ מִח֥וּץ לַֽמַּחֲנֶ֖ה שִׁבְעַ֣ת יָמִ֑ים כֹּל֩ הֹרֵ֨ג נֶ֜פֶשׁ וְכֹ֣ל ׀ נֹגֵ֣עַ בֶּֽחָלָ֗ל תִּֽתְחַטְּא֞וּ בַּיּ֤וֹם הַשְּׁלִישִׁי֙ וּבַיּ֣וֹם הַשְּׁבִיעִ֔י אַתֶּ֖ם וּשְׁבִיכֶֽם׃