Romans 9:7-9

9:7 nor are all the children Abraham’s true descendants; rather “through Isaac will your descendants be counted.” 9:8 This means it is not the children of the flesh who are the children of God; rather, the children of promise are counted as descendants. 9:9 For this is what the promise declared: “About a year from now I will return and Sarah will have a son.”

Galatians 4:22-26

4:22 For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. 4:23 But one, the son by the slave woman, was born by natural descent, while the other, the son by the free woman, was born through the promise. 4:24 These things may be treated as an allegory, for these women represent two covenants. One is from Mount Sinai bearing children for slavery; this is Hagar. 4:25 Now Hagar represents Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children. 4:26 But the Jerusalem above is free, and she is our mother.