Numbers 31:39

And the asses were thirty thousand and five hundred; of which the LORD’s tribute was threescore and one.

The verse belongs to the account of Israel's first taking of captives, when Israel marched into Midian, slew all the males, and seized the women, children, flocks and herds. After Moses reprimanded the army for sparing the grown women, all were put to death except the virgins and children. The verse records part of the division of that spoil after the defeat of the Midianites, which included people, cattle, sheep, asses and other animals, with one out of every five hundred persons, beeves, asses and sheep set apart.

The word for soul is applied indiscriminately to both the human beings and the lower animals captured in the war, so the asses numbered here are counted as souls just as the persons are. The surviving virgins and children were reserved for adoption into the families of the nation, not for slavery, and a captive woman taken and humbled could not afterward be sold for money or made merchandise of even if repudiated. By thus prohibiting the enslavement of prisoners of war, the most productive mode of slave-making, God positively prohibited Israel from becoming a slaveholding nation.