פירוש על קידושין 57:12

Him and not her- If you will say why do we need the passuk? For time bound mitzvoth we know women are exempt. From the eighth day on its a mitzvah until you do it. Since its a mitzvah can we do it during the day or at night? From the eighth day on if you haven't done it yet you can do it at night

God commanded Abraham to circumcise Yitzchak. He did not command Sarah.