Leviticus 25:46 allows the Israelites to take slaves from the surrounding nations, to leave them to their sons as property, and to hold them as slaves for ever, while forbidding Israelites to rule over one another ruthlessly. The argument says this means slavery is something the Bible sanctions as God’s own law, passed down in families and lasting for life.
You may bequeath them to your sons after you to inherit as a possession forever. You may make slaves of them, but over your brothers the people of Israel you shall not rule, one over another ruthlessly.
Leviticus 25:46
Source: see Credits and references (no. 38, p. 30).