פירוש על בראשית 21:4

וימל...כאשר צוה אותו אלוקים, compare 17,12 that any son should be circumcised when eight days old. Our sages explain that the word אותו, means that only the father is commanded to perform circumcision on his son, not the mother.

As He had commanded: [He commanded] the laws of circumcision orally; as it is found in Berakhot 5 (a), "'and the commandment' (Exodus 24:12) - that is the Mishnah (meaning the oral teaching)." There were several commandments like this to Avraham, our father, such as [doing the circumcision] specifically during the day, and not at the beginning of the night of the eighth day.

בן שמונת ימים, “when eight days old;” Nowadays (author’s time) the parents are in the habit of bringing the baby to the Synagogue where he is welcomed with the words: ברוך הבא, “blessed be the new arrival.” The reason why this formula has become popular is that the numerical value of the word הבא=8. The benediction therefore is meant to say: “blessed be he who is being circumcised on the eighth day.”