Sarai, Avram’s wife, had borne him no children. She had an Egyptian maidservant whose name was Hagar.
Rashi on Genesis 16:1 — Rashi explicitly cites Bereshit Rabbah 45:1 for the tradition that Hagar was Pharaoh’s daughter.
English: “A handmaid, an Egyptian” — she was Pharaoh’s daughter; when he saw the miracles done for Sarah, he said: Better that my daughter be a handmaid in this house than a mistress in another house.
Supplementary classical material not listed by Torah Temimah under this exact passuk.