Genesis 24:4

“For putting his hand under the thigh of a man, and swearing by the God of Heaven, what else did he signify, than that in that Flesh, which derived its origin from that thigh, the God of Heaven would come?” — editor's note: Gen. xxiv. 2–4

“But that the servant might apprehend what Abraham knew, that it was not offspring after the flesh he desired, nor anything of a carnal kind concerning his race that was referred to, he said to the servant whom he sent, “Put thy hand under my thigh, and swear by the God of heaven.”” — editor's note: Gen. xxiv. 2–4.

Cited together in On the Good of Marriage Chapter 22.