Galatians 4:20-24

but I could wish to be present with you now and to change my tone of voice, for I am at a loss about you! Slave and Free

Tell me, you who want to be under law, do you not listen to the Law?

For it is written that Abraham had two sons, one by the slave woman and one by the free woman.

But the son by the slave woman was born according to the flesh, and the son by the free woman through the promise.

This is speaking allegorically, for these women are two covenants: one coming from Mount Sinai giving birth to children who are to be slaves; she is Hagar.