Dora Bas Rivka Silver O'H
The Mitzvah of Milah
G-d now told Avraham the details of His covenant with his descendants: G-d would be with them throughout the generations, forever, and they would own the land of Canaan. As a sign of this covenant, all males must be circumcised by removing the foreskin of the penis. This is an immutable part of the contract, carrying the penalty of excision for noncompliance (which is almost unprecedented in the case of failing to perform a positive mitzvah). This mitzvah is called bris milah, the covenant of circumcision.
G-d changed Sarai's name to Sarah and promised that she would bear a son and be the mother of nations. Avraham laughed in joy at the thought of Sarah and he having children at the ages of 90 and 100, respectively. But Avraham was worried how this would affect Yishmael, his son with Hagar. G-d assured him that Yishmael would also be built into a great nation, but G-d's covenant with Avraham would be fulfilled through his son with Sarah, whom they should name Yitzchak (Isaac). With that, G-d "left." (That is, the vision ended.)
That very day, Avraham circumcised himself and all the males in his household, including Yishmael, who was then 13 years old.
One question: we will see in parshas Toldos (Gen. 26:5) that Avraham figured out and kept all the mitzvos, even though they had not yet been commanded. If so, why did he not circumcise himself until G-d commanded it? The answer is that there is a prohibition against self-injury; Avraham would not have been permitted to inflict such a wound upon himself without G-d's specific instruction to do so.