Parashas Toldos · Passuk 25:33
But Yaakov said, “Swear to me as this day.” So he swore to him, and sold his birthright to Yaakov.
Ramban on Bereishis 25:33 (classical source on this passuk; not listed as a numbered Torah Temimah unit here).
Lemma / focus: השבעה לי כיום.
English: When Esav said, “Why do I need a birthright? I do not desire it,” Yaakov said to him, “Swear to me that you will not desire it and will not inherit it forever.”
Classical commentary on the legal and narrative force of the oath; not a formal named middah of the 13.
Ramban explains that Yaakov required Esav to swear that he would not later claim or inherit the birthright after declaring that he did not want it.