Cross reference
Galatians 4:22
22 Because it is in the Writings, that Abraham had two sons, one by the servant-woman, and one by the free woman.
Genesis 16:15
15 And Hagar gave birth to a child, the son of Abram, to whom Abram gave the name of Ishmael.
Genesis 16:2-4
2 And Sarai said to Abram, See, the Lord has not let me have children; go in to my servant, for I may get a family through her. And Abram did as Sarai said.
3 So after Abram had been living for ten years in the land of Canaan, Sarai took Hagar, her Egyptian servant, and gave her to Abram for his wife.
4 And he went in to Hagar and she became with child, and when she saw that she was with child, she no longer had any respect for her master's wife.
Genesis 21:1-2
1 And the Lord came to Sarah as he had said and did to her as he had undertaken.
2 And Sarah became with child, and gave Abraham a son when he was old, at the time named by God.
Genesis 21:10
10 So she said to Abraham, Send away that woman and her son: for the son of that woman is not to have a part in the heritage with my son Isaac.