Albert Barnes Commentary Galatians 4:22

"For it is written, that Abraham had two sons, one by the handmaid, and one by the freewoman." — Galatians 4:22 (ASV)

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 25:1–6). 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).