Numbers 31:19
And do ye abide without the camp seven days: whosoever hath killed any person, and whosoever hath touched any slain, purify both yourselves and your captives on the third day, and on the seventh day.
And do ye abide without the camp seven days..... Which was the time that anyone that touched a dead body remained unclean,
whosoever hath killed any person; as most of them if not all must have done; all the males of Midian that fell into their hands being slain by them, that were men grown:
and whosoever hath touched any slain; as they must to strip them of their garments, and take their spoil from them:
purify both yourselves and your captives, on the third and on the seventh day; which were the days appointed for the purification of such that were polluted by touching dead bodies, Numbers 19:11 and their captives, which were the female little ones; (for as for the women, and males among the little ones, they were ordered to be slain;) though they were Heathens, yet inasmuch as they were to be for the service of the Israelites, and to be brought up in their religion, they were to be purified also; to which purpose is the note of Jarchi;
"not that the Gentiles receive uncleanness and need sprinkling, but as ye are the children of the covenant, so your captives, when they come into the covenant, and are defiled, need sprinkling.''