23 However, the son by the servant was born according to the flesh, but the son by the free woman was born through promise.
Galatians 4:23 Cross References - WEB
Genesis 17:15-19
15 God said to Abraham, “As for Sarai your wife, you shall not call her name Sarai, but her name will be Sarah.
16 I will bless her, and moreover I will give you a son by her. Yes, I will bless her, and she will be a mother of nations. Kings of peoples will come from her.”
17 Then Abraham fell on his face, and laughed, and said in his heart, “Will a child be born to him who is one hundred years old? Will Sarah, who is ninety years old, give birth?”
18 Abraham said to God, “Oh that Ishmael might live before you!”
19 God said, “No, but Sarah, your wife, will bear you a son. You shall call his name Isaac. I will establish my covenant with him for an everlasting covenant for his offspring after him.
Genesis 18:10-14
10 He said, “I will certainly return to you at about this time next year; and behold, Sarah your wife will have a son.” Sarah heard in the tent door, which was behind him.
11 Now Abraham and Sarah were old, well advanced in age. Sarah had passed the age of childbearing.
12 Sarah laughed within herself, saying, “After I have grown old will I have pleasure, my lord being old also?”
13 Yahweh said to Abraham, “Why did Sarah laugh, saying, ‘Will I really bear a child when I am old?’
14 Is anything too hard for Yahweh? At the set time I will return to you, when the season comes round, and Sarah will have a son.”
Genesis 21:1-2
Romans 4:18-21
18 Besides hope, Abraham in hope believed, to the end that he might become a father of many nations, according to that which had been spoken, “So will your offspring be.”
19 Without being weakened in faith, he didn’t consider his own body, already having been worn out, (he being about a hundred years old), and the deadness of Sarah’s womb.
20 Yet, looking to the promise of God, he didn’t waver through unbelief, but grew strong through faith, giving glory to God,
21 and being fully assured that what he had promised, he was also able to perform.
Romans 9:7-8
Romans 10:8
8 But what does it say? “The word is near you, in your mouth, and in your heart;” that is, the word of faith which we preach:
Galatians 4:28-29
Hebrews 11:11
11 By faith, even Sarah herself received power to conceive, and she bore a child when she was past age, since she counted him faithful who had promised.