Galatians 4:22-26

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.

Now this Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is enslaved with her children.

But the Jerusalem above is free; she is our mother.