Komentarz do Rodzaju 21:5

ואברהם בן מאת שנה, although as an old man we could have expected Avraham to delegate the task of performing his son’s circumcision, he did so himself, ignoring the fact that he was an old man. [the commentary is based on the fact that we knew that Avraham was 100 years old at the time, so why repeat it?]

ואברהם...את יצחק בנו, this verse is another example of the word את appearing in conjunction with verbs which appear in the passive mode. To name just a few such examples: Numbers 26,55 יחלק את הארץ, or Deut. 20,5 ולא ימס את לבב אחיו. What is the reason that the Torah saw fit to write this verse at all? Seeing that the Torah had told us that Avraham had been 99 years old when he had received the commandment of circumcision, and G’d had told him that Sarah would bear this son for him in the following year, we knew that he would be 100 years old at that time! (17,21) Perhaps the Torah wanted to tell us that Avraham had completed 100 years when Yitzchok was born and that the previous prophecy had been revealed to him when he had completed 99 years. In that event, Sarah’s pregnancy lasted fully 12 months, so that the news revealed in our verse is that her pregnancy was unnaturally lengthy.

And Avraham was a hundred years old: But it is no longer written, "and Sarah was ninety years old'; for behold, she had already reverted to being a youngster.