Genesis 24:3

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:

Thou shalt not take a wife, ... Among the pastoral tribes the matrimonial arrangements are made by the parents, and a youth must marry, not among strangers, but in his own tribe-custom giving him a claim, which is seldom or never resisted, to the hand of his first cousin. But Abraham had a far higher motive-a fear lest, if his son married into a Canaanitish family, he might be gradually led away from the true God; and hence, as that people were gross idolaters, he was desirous that a matrimonial negotiation should be opened with his Mesopotamian relatives, who, amidst some corruptions, still retained the knowledge and worship of God.

Said unto his oldest servant. Abraham, being too old, and, as the heir of the promise not being at liberty, to make even a temporary visit to his native land, was obliged to entrust this delicate mission to "his oldest servant of his house," whom, although putting entire confidence in him, he on this occasion bound by solemn oath. A pastoral chief in the present day would take the same plan, if he could not go himself-that of sending a confidential servant, with or without his son, in search of a wife; and the servant has power to settle the affair, and bring home the bride. It is commonly supposed that this person was the steward, over-seer, major-domo, of the house and all the servants of the establishment; but such a confidential officer is designated in this book by a different phraseology (see Genesis 15:2; Genesis 43:16; Genesis 44:1). The nature of the mission on which he was to be employed requiring both experience and judgment, it is probable that the words are used here in their literal sense, to denote superiority in age. Had Eliezer been the servant, his name would doubtless have been mentioned; and as sixty years had elapsed since Abraham received the divine promise of offspring (Genesis 15:4), and Isaac was now forty years old (Genesis 25:20), it may well be presumed that Eliezer, if he had ever entertained hopes of the inheritance, must, have so completely dismissed them as to justify his employment on the delicate negotiation of Isaac's marriage. But reasons were assigned for thinking that Eliezer was not a servant in Abraham's house (see the note at Genesis 15:2).

Put ... thy hand under my thigh. This form of oath, which was peculiar to the patriarchs (cf. 47:29), referred to the solemn covenant of circumcision by which the Hebrews were separated from all other people to the service and fellowship of the one living and true God; it recognized also their national hope of abundant fruitfulness (cf. Genesis 46:26), and that, through Abraham's seed, all kindreds of the earth should be blessed. The asseveration was to be by "Yahweh, the God of heaven and the God of the earth;" - the reason being, that the embassy on which the servant was about to be despatched was not a common matrimonial negotiation, but one which had an important prospective reference to the kingdom of God.