Cross reference
Galatians 4:22
22 For it is written that Abraham had two sons, one by the handmaid, and one by the free woman.
Genesis 16:15
15 Hagar bore a son for Abram. Abram called the name of his son, whom Hagar bore, Ishmael.
Genesis 16:2-4
2 Sarai said to Abram," See now, Yahweh has restrained me from bearing. Please go in to my handmaid. It may be that I will obtain children by her." Abram listened to the voice of Sarai.
3 Sarai, Abram's wife, took Hagar the Egyptian, her handmaid, after Abram had lived ten years in the land of Canaan, and gave her to Abram her husband to be his wife.
4 He went in to Hagar, and she conceived. When she saw that she had conceived, her mistress was despised in her eyes.
Genesis 21:1-2
1 Yahweh visited Sarah as he had said, and Yahweh did to Sarah as he had spoken.
2 Sarah conceived, and bore Abraham a son in his old age, at the set time of which God had spoken to him.
Genesis 21:10
10 Therefore she said to Abraham, "Cast out this handmaid and her son! For the son of this handmaid will not be heir with my son, even with Isaac."