Genesis 24:4 (KJV)

And Abraham was old, and well stricken in age: and the LORD had blessed Abraham in all things.

And Abraham said unto his eldest servant of his house, that ruled over all that he had, Put, I pray thee, thy hand under my thigh:

And I will make thee swear by the LORD, the God of heaven, and the God of the earth, that thou shalt not take a wife unto my son of the daughters of the Canaanites, among whom I dwell:

And the servant said unto him, Peradventure the woman will not be willing to follow me unto this land: must I needs bring thy son again unto the land from whence thou camest?

And Abraham said unto him, Beware thou that thou bring not my son thither again.

The LORD God of heaven, which took me from my father's house, and from the land of my kindred, and which spake unto me, and that sware unto me, saying, Unto thy seed will I give this land; he shall send his angel before thee, and thou shalt take a wife unto my son from thence.

And if the woman will not be willing to follow thee, then thou shalt be clear from this my oath: only bring not my son thither again.

And the servant put his hand under the thigh of Abraham his master, and sware to him concerning that matter.