For it is written - Genesis 16; Genesis 21.
Abraham had two sons - Ishmael and Isaac. Abraham subsequently had several sons by Keturah after the death of Sarah; Genesis 26:1-6. But the two sons by Hagar and Sarah were the most prominent, and the events of their lives furnished the particular illustration which Paul desired.
The one by a bond-maid - Ishmael, the son of Hagar. Hagar was an Egyptian slave, whom Sarah gave to Abraham in order that he might not be wholly without posterity; Genesis 16:3.
The other by a free woman - Isaac, the son of Sarah; Genesis 21:1-2.