Rashi on Genesis 24:2/רש"י על בראשית כ״ד:ב׳

1זקן ביתו THE ELDER OF HIS HOUSE — Because the noun is in the construct state the ק is punctuated with a Patach.

2תחת ירכי UNDER MY THIGH — because whoever takes an oath must take in his hand some sacred object. such as a Scroll of the Law or Tefillin (Shevuot 38b). As circumcision was the first commandment given to him and became his only through much pain it was consequently dear to him and therefore he selected this as the object upon which to take the oath (Genesis Rabbah 59:8).