משך חכמה — והוא אחריו

Q — To whom does the cryptic phrase "והוא אחריו" refer, and why is the parenthetical "ושרה שומעת פתח האהל" inserted in the middle of the angel's announcement?

A — "והוא אחריו" is part of the angel's own words — "the son is Yitzchak, and he is Avraham's true 'after,'" the fulfillment of the prior promise of "זרעך אחריך," to the exclusion of Yishmael and the sons of Keturah; the verse switched mid-sentence from speaking to Avraham (which would need "אחריך") to speaking through Sarah's listening, using the rule of "מוקדם שהוא מאוחר בענין."