ואברהם בן תשעים ותשע שנה בהמלו בשר ערלתו (לך יז:כד)

Chazal says (קידושין פב) that Avraham kept all the Torah’s commandments – even before the Torah was given to the Yidden on Mount Sinai.  So why did he wait until he was ninety-nine years of age for Hashem to command him to perform a Bris Milah?

This question was posed to the Vilna Gaon Zt”l by the elders of his town when he was just five years old. He promptly replied: Chazal say (קידושין לא) that “גדול המצווה ועושה” it is more significant for one to perform a mitzvah because he was commanded to do, than one who performs a mitzvah that he was not commanded.  Thus, Avraham could perform all mitzvahs before he was commanded to do so, with the hope that if and when he would be commanded to perform a mitzvah, he could do it all over.  Not so with the mitzvah of Bris Milah, which – once performed – cannot be repeated.   Avraham, therefore, waited for Hashem to command him to circumcise himself so that he would be able to be a “מצווה ועושה.”