Genesis 12

 We are told that while in Egypt, because of Sarai, that Pharaoh -

"...entreated Abram well for her sake: and he had sheep, and oxen, and he asses, and menservants, and maidservants, and she asses, and camels."  (Genesis 12:16)

But also, Josephus recorded of Abraham - 

"...He communicated to them arithmetic, and delivered to them the science of astronomy; for before Abram came into Egypt they were unacquainted with those parts of learning..." (Josephus, Flavius, Antiquities of the Jews, Book 1, chapter 8, verses 1– 2).

This note seems to confirm the authenticity of the Book of Abraham