From here [we learn] that one who swears must hold a sacred object in his hand.
On the daf / in context: The requirement that a person taking an oath grasp a sacred object is derived from Avraham’s instruction to Eliezer to place his hand under the thigh (the site of the milah).
Type of Learning
Custom (Bavli — holding a sacred object for an oath)
What Is Learned
An oath requires the swearer to grasp a sacred object. Avraham’s demand that Eliezer place his hand under the thigh is the Torah’s paradigm for this practice.