In Genesis 24:2–9 Abraham has his servant Eliezer put his hand under the Patriarch’s thigh to swear “by the Lord, the God of heaven and earth” that the servant will not arrange a marriage for Abraham’s son Isaac with a Canaanite woman. Similarly, in Genesis 47:29–31 the dying Patriarch Jacob has his son Joseph swear to him that he will bury Jacob not in Egypt, but alongside Jacob’s own parents in the Cave of Machpelah; and the oath-taking ritual again calls for putting a hand under the Patriarch’s thigh.
Talmudic tradition1 takes these verses to indicate that the oath was sworn while the circumcised membrum of the Patriarch was held in hand, and derives from this interpretation the rule that all Jewish oaths must be sworn while some ritual object is held in hand. Ordinary people must hold a Torah scroll; scholars may hold any ritual object.
The reason the Talmud requires an oath to be sworn while a ritual object is held is indicated by such sources as Genesis 21:23, where Abraham is required by Abimelech to swear “by God (or ‘gods’?) here”; and Exodus 17:16, where Moses seems to be swearing an oath by the divine throne. The ritual object is supposed to indicate the presence of God at the time of the swearing. Thus the gravity of the oath is impressed upon the one who swears it.