Numbers 31:18

“But all the women children, etc.] The females among the little ones: that have not known a man by lying with him; which might be pretty clear, and easily concluded, from their age: keep for yourselves; either to be handmaids to them, or to be married among them when grown up, and become proselytes, and initiated into their religion.”

“keep alive for yourselves: It has been groundlessly asserted, that Moses here authorised the Israelites to make concubines of the whole number of female children; and an insidious objection against his writings has been grounded upon this monstrous supposition. But the whole tenor of the law, and especially a statute recorded in Deu 21:10-14, proves most decisively to the contrary. They were merely permitted to possess them as female slaves, educating them in their families, and employing them as domestics; for the laws concerning fornication, concubinage, and marriage, were in full force, and prohibited an Israelite even from marrying a captive, without delays and previous formalities; and…”